GSS-API / SSPI authenticated Diffie-Hellman Key Exchange and user authentication

This patch implements the GSS-API / SSPI authenticated Diffie-Hellman
Key Exchange and user authentication (gssapi-keyex) with the context
established during key exchange.
In order to use this feature you need the following packages:
- pyasn1 0.1.7
- python-gssapi 0.4.0 (Unix)
- pywin32 2.1.8 (Windows)

These packages are not required for Paramiko, but to use GSS-API / SSPI
authenticated key exchange you need to install them manually.
This patch also contains the patch of pull request #228 which implements
"diffie-hellman-group14-sha1" key exchange.