Add primary user id signature subpacket parsing to PGPPacket

Add primary user id signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (1 octet, Boolean)

  This is a flag in a User ID's self-signature that states whether
  this User ID is the main User ID for this key.  It is reasonable for
  an implementation to resolve ambiguities in preferences, etc. by
  referring to the primary User ID.  If this flag is absent, its value
  is zero.  If more than one User ID in a key is marked as primary,
  the implementation may resolve the ambiguity in any way it sees fit,
  but it is RECOMMENDED that priority be given to the User ID with the
  most recent self-signature.

  When appearing on a self-signature on a User ID packet, this
  subpacket applies only to User ID packets.  When appearing on a
  self-signature on a User Attribute packet, this subpacket applies
  only to User Attribute packets.  That is to say, there are two
  different and independent "primaries" -- one for User IDs, and one
  for User Attributes.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.19