Add key expiration time subpacket parsing to PGPPacket

Add key expiration time subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (4-octet time field)

  The validity period of the key.  This is the number of seconds after
  the key creation time that the key expires.  If this is not present
  or has a value of zero, the key never expires.  This is found only
  on a self-signature.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.6