GSS-API / SSPI Authentication

This patch implements the GSS-API user authentication (gssapi-with-mic)
for Paramiko as defined in RFC 4462 Section 3.
GSS-API Key Exchange (RFC 4462, Section 4) is not implemented yet.
New optional packages for Paramiko:
- pyasn1 0.1.7
- python-gssapi 0.4.0 (Unix)
- pywin32 2.1.8 (Windows)

These packages are not required for Paramiko, but if you want to use GSS-API / SSPI authentication you need to install them manually.