Add PGPPacket._signature_types

Add PGPPacket._signature_types

From RFC 4880 [1]:

  There are a number of possible meanings for a signature, which are
  indicated in a signature type octet in any given signature.  Please
  note that the vagueness of these meanings is not a flaw, but a
  feature of the system.  Because OpenPGP places final authority for
  validity upon the receiver of a signature, it may be that one
  signer's casual act might be more rigorous than some other
  authority's positive act.  See Section 5.2.4, "Computing
  Signatures", for detailed information on how to compute and verify
  signatures of each type.

  These meanings are as follows:

  0x00: Signature of a binary document.
    This means the signer owns it, created it, or certifies that it
    has not been modified.

  0x01: Signature of a canonical text document.
    This means the signer owns it, created it, or certifies that it
    has not been modified.  The signature is calculated over the text
    data with its line endings converted to <CR><LF>.

  0x02: Standalone signature.
    This signature is a signature of only its own subpacket contents.
    It is calculated identically to a signature over a zero-length
    binary document.  Note that it doesn't make sense to have a V3
    standalone signature.

  0x10: Generic certification of a User ID and Public-Key packet.
    The issuer of this certification does not make any particular
    assertion as to how well the certifier has checked that the owner
    of the key is in fact the person described by the User ID.

  0x11: Persona certification of a User ID and Public-Key packet.
    The issuer of this certification has not done any verification of
    the claim that the owner of this key is the User ID specified.

  0x12: Casual certification of a User ID and Public-Key packet.
    The issuer of this certification has done some casual verification
    of the claim of identity.

  0x13: Positive certification of a User ID and Public-Key packet.
    The issuer of this certification has done substantial verification
    of the claim of identity.

    Most OpenPGP implementations make their "key signatures" as 0x10
    certifications.  Some implementations can issue 0x11-0x13
    certifications, but few differentiate between the types.

  0x18: Subkey Binding Signature
    This signature is a statement by the top-level signing key that
    indicates that it owns the subkey.  This signature is calculated
    directly on the primary key and subkey, and not on any User ID or
    other packets.  A signature that binds a signing subkey MUST have
    an Embedded Signature subpacket in this binding signature that
    contains a 0x19 signature made by the signing subkey on the
    primary key and subkey.

  0x19: Primary Key Binding Signature
    This signature is a statement by a signing subkey, indicating that
    it is owned by the primary key [2].  This signature is calculated
    the same way as a 0x18 signature: directly on the primary key and
    subkey, and not on any User ID or other packets.

  0x1F: Signature directly on a key
    This signature is calculated directly on a key.  It binds the
    information in the Signature subpackets to the key, and is
    appropriate to be used for subpackets that provide information
    about the key, such as the Revocation Key subpacket.  It is also
    appropriate for statements that non-self certifiers want to make
    about the key itself, rather than the binding between a key and a
    name.

  0x20: Key revocation signature
    The signature is calculated directly on the key being revoked.  A
    revoked key is not to be used.  Only revocation signatures by the
    key being revoked, or by an authorized revocation key, should be
    considered valid revocation signatures.

  0x28: Subkey revocation signature
    The signature is calculated directly on the subkey being revoked.
    A revoked subkey is not to be used.  Only revocation signatures by
    the top-level signature key that is bound to this subkey, or by an
    authorized revocation key, should be considered valid revocation
    signatures.

  0x30: Certification revocation signature
    This signature revokes an earlier User ID certification signature
    (signature class 0x10 through 0x13) or direct-key signature
    (0x1F).  It should be issued by the same key that issued the
    revoked signature or an authorized revocation key.  The signature
    is computed over the same data as the certificate that it revokes,
    and should have a later creation date than that certificate.

  0x40: Timestamp signature.
    This signature is only meaningful for the timestamp contained in
    it.

  0x50: Third-Party Confirmation signature.
    This signature is a signature over some other OpenPGP Signature
    packet(s).  It is analogous to a notary seal on the signed data.
    A third-party signature SHOULD include Signature Target
    subpacket(s) to give easy identification.  Note that we really do
    mean SHOULD.  There are plausible uses for this (such as a blind
    party that only sees the signature, not the key or source
    document) that cannot include a target subpacket.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.1
[2]: http://www.rfc-editor.org/errata_search.php?rfc=4880
     Errata ID: 2208
     Reported By: Constantin Hagemeier
     Date Reported: 2010-04-28
     Held for Document Update by: Sean Turner
     Date Held: 2010-07-20

     Section 5.2.1. says:

       This signature is a statement by a signing subkey, indicating
       that it is owned by the primary key and subkey.

     It should say:

       This signature is a statement by a signing subkey, indicating
       that it is owned by the primary key.

     Notes:

       The subkey does not own itself.