Add proxy_connect convenience function to SSHClient

I would like to propose a convenience function for chaining ssh connections using a function in the SSHClient class. I'd propose something like the following:

def proxy_connect(self, hostname, port=SSH_PORT, **kwargs):
        """
        Connect to an SSH server through the currently open connection. Keyword arguments
        are the same as those for connect. 

        Note that the missing host key policy is coppied from the current client.

        @return: A new SSHClient connection to the host. 
        @rtype: L{SSHClient}
        """
        transport = self.get_transport()
        kwargs['sock'] = transport.open_channel('direct-tcpip', (hostname, port), ('', 0))
        ssh = SSHClient()
        ssh.set_missing_host_key_policy(self._policy)
        ssh.connect(hostname, port, **kwargs)
        return ssh

Start in the SSHClient implementation and trace get_transport(), open_channel(), connect(), and the missing-host-key policy. Review existing SSH connection behavior before adding the convenience entry point, then verify that it returns a usable client for a chained connection and preserves the current policy.