Add preferred compression algorithm signature subpacket parsing to PGPPacket

Add preferred compression algorithm signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (array of one-octet values)

  Compression algorithm numbers that indicate which algorithms the key
  holder prefers to use.  Like the preferred symmetric algorithms, the
  list is ordered.  Algorithm numbers are in Section 9.  If this
  subpacket is not included, ZIP is preferred.  A zero denotes that
  uncompressed data is preferred; the key holder's software might have
  no compression software in that implementation.  This is only found
  on a self-signature.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.9