Add PGPPacket._str_signature_packet

Add PGPPacket._str_signature_packet

This is not super useful without parsing the subpackets, but you can
usually figure out what's being bound using the context:

  $ ./gpg-migrate.py F15F5BE8 6D024CA2
  key: F15F5BE8
    public:
      public-key packet: F15F5BE8
      user id packet: William Trevor King <wking@tremily.us>
      signature packet: postitive user id and public-key packet
      signature packet: postitive user id and public-key packet
      ...
      user id packet: William Trevor King <tvrkng@gmail.com>
      signature packet: postitive user id and public-key packet
      signature packet: postitive user id and public-key packet
      signature packet: postitive user id and public-key packet
      signature packet: generic user id and public-key packet
      signature packet: postitive user id and public-key packet
      public-subkey packet: 42407C74
      signature packet: subkey binding
  ...

The user id signatures are likely between the previous user id and
F15F5BE8, with positive self-signed signatures and generic signatures
from other folks.  The subkey binding is likely between F15F5BE8 and
42407C74.  Adding subpacket parsing to make this explicit would be
nice, but it's not a priority at the moment.