Add features signature subpacket parsing to PGPPacket

Add features signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

 (N octets of flags)

 The Features subpacket denotes which advanced OpenPGP features a
 user's implementation supports.  This is so that as features are
 added to OpenPGP that cannot be backwards-compatible, a user can
 state that they can use that feature.  The flags are single bits that
 indicate that a given feature is supported.

 This subpacket is similar to a preferences subpacket, and only
 appears in a self-signature.

 An implementation SHOULD NOT use a feature listed when sending to a
 user who does not state that they can use it.

 Defined features are as follows:

   First octet:

   0x01 - Modification Detection (packets 18 and 19)

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.24