PGPMessage.decrypt should offer the ability to decrypt with a session key

[dkg]

Currently, PGPMessage.decrypt() allows decryption with a passphrase, expecting an SKESK packet in the message to yield a session key.

But it's possible that the user already has a session key handy, and would like to use it instead of a passphrase (or in the event that no SKESK packet is present).

It would be nice if PGPy could support this use case cleanly and simply.