banner functionality

I would like to be able to obtain the banner supplied by a server to which my python script connects using paramiko.
This is to be able to supply reasons for not be able to connect at a certain moment, (can be handy for legal issues as well)
The following changes are in line wich the get_username method.
Tested and works. :-)
I have not updated documentation in the pull request but this line can be added I suppose in the class Transport documentation on http://www.lag.net/paramiko/docs/ :
string get_banner(self)
            Return the banner supplied by the remote server.