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.
I've created a new branch to fix that issue, but i haven't had much time to test it.
See https://github.com/SebastianDeiss/paramiko/tree/python-gssapi-0.6.1