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.
When merging this we should probably reorder so we have diffie-hellman-group14-sha1 as preferred kex. The default one now: diffie-hellman-group1-sha1 is no longer considered secure. It is required to support it according to the rfc, but the order is not stated. I think we should reorder them as such:
```
    _preferred_kex = ( 'diffie-hellman-group14-sha1', 'diffie-hellman-group-exchange-sha1' , 'diffie-hellman-group1-sha1')
```