Adding SSH Agent forwarding basic support (again)

Same thing in closed pull request : https://github.com/robey/paramiko/pull/14

This kind of feature can be usefull for library like Fabric to allow remote git pull with forwarded ssh-agent : http://code.fabfile.org/issues/show/72

But its hard to test an proxyfied ssh-agent connexion.
If tests are really needed and some other people know how to test this, feel free to improve this pull request...