Add key flags signature subpacket parsing to PGPPacket

Add key flags signature subpacket parsing to PGPPacket

From RFC 4880 [1]:

  (N octets of flags)

  This subpacket contains a list of binary flags that hold information
  about a key.  It is a string of octets, and an implementation MUST
  NOT assume a fixed size.  This is so it can grow over time.  If a
  list is shorter than an implementation expects, the unstated flags
  are considered to be zero.  The defined flags are as follows:

    First octet:

    0x01 - This key may be used to certify other keys.
    0x02 - This key may be used to sign data.
    0x04 - This key may be used to encrypt communications.
    0x08 - This key may be used to encrypt storage.
    0x10 - The private component of this key may have been split by a
           secret-sharing mechanism.
    0x20 - This key may be used for authentication.
    0x80 - The private component of this key may be in the possession
           of more than one person.

[1]: http://tools.ietf.org/search/rfc4880#section-5.2.3.21