Add version-4-signature parsing to PGPPacket

Add version-4-signature parsing to PGPPacket

From RFC 4880 [1]:

  The body of a version 4 Signature packet contains:

  - One-octet version number (4).
  - One-octet signature type.
  - One-octet public-key algorithm.
  - One-octet hash algorithm.
  - Two-octet scalar octet count for following hashed subpacket data.
    Note that this is the length in octets of all of the hashed
    subpackets; a pointer incremented by this number will skip over
    the hashed subpackets.
  - Hashed subpacket data set (zero or more subpackets).
  - Two-octet scalar octet count for the following unhashed subpacket
    data.  Note that this is the length in octets of all of the
    unhashed subpackets; a pointer incremented by this number will
    skip over the unhashed subpackets.
  - Unhashed subpacket data set (zero or more subpackets).
  - Two-octet field holding the left 16 bits of the signed hash value.
  - One or more multiprecision integers comprising the signature.
    This portion is algorithm specific, as described above.

I've stashed all of the algorithm-specific MPI values in the
'signature' field, because I don't care about verifying signatures at
the moment.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3