'Send a USER command to perform the first half of plaintext login.
Unless this is absolutely required, use the L{login} method instead.
@type username: L{bytes}
@param username: The username with which to log in.
@rtype: L{Deferred <defer.Deferred>} which successfully fires with
L{bytes} or fails with L{ServerErrorResponse}
@return: A deferred which fires when the server response is received.
On an OK response, the deferred succeeds with the server
response minus the status indicator.  On an ERR response, the
deferred fails with a server error response failure.'

def user(self, username):

    return self.sendShort('USER', username)
