Add preferred symmetric algorithm signature subpacket parsing to PGPPacket

Add preferred symmetric algorithm signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (array of one-octet values)

  Symmetric algorithm numbers that indicate which algorithms the key
  holder prefers to use.  The subpacket body is an ordered list of
  octets with the most preferred listed first.  It is assumed that
  only algorithms listed are supported by the recipient's software.
  Algorithm numbers are in Section 9.  This is only found on a self-
  signature.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.7