GSS-API / SSPI authenticated Diffie-Hellman Key Exchange and user authentication with Python 3 support

Add Python 3 support for the GSS-API / SSPI authenticated Diffie-Hellman
Key Exchange and user authentication.
This patch supersedes pull request #250.
This is still on my milestone list for 1.15 and I've been trying to catch this sort of thing as I go. Good looking out tho.