Add public key parsing to PGPPacket

Add public key parsing to PGPPacket

Use the same parser for public-key and public-subkey packets.  From
RFC 4880 [1]:

  A Public-Subkey packet (tag 14) has exactly the same format as a
  Public-Key packet, but denotes a subkey.  One or more subkeys may be
  associated with a top-level key.

The generic (sub)key parsing is specified in section 5.5.2 [2]:

  OpenPGP implementations MUST create keys with version 4 format.  V3
  keys are deprecated; an implementation MUST NOT generate a V3 key,
  but MAY accept it.

  ...

  A version 4 packet contains:

  - A one-octet version number (4).
  - A four-octet number denoting the time that the key was created.
  - A one-octet number denoting the public-key algorithm of this key.
  - A series of multiprecision integers comprising the key material.

Also check that the --export packets begin with a public-key packet.
From RFC 4880 [3]:

  A Public-Key packet starts a series of packets that forms an OpenPGP
  key (sometimes called an OpenPGP certificate).

[1]: http://tools.ietf.org/search/rfc4880#section-5.5.1.2
[2]: http://tools.ietf.org/search/rfc4880#section-5.5.2
[3]: http://tools.ietf.org/search/rfc4880#section-5.5.1.1