Add preferred hash algorithm signature subpacket parsing to PGPPacket

Add preferred hash algorithm signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (array of one-octet values)

  Message digest algorithm numbers that indicate which algorithms the
  key holder prefers to receive.  Like the preferred symmetric
  algorithms, the list is ordered.  Algorithm numbers are in Section
  9.  This is only found on a self-signature.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.8