'Perform a plaintext login.
@type username: L{bytes}
@param username: The username with which to log in.
@type password: L{bytes}
@param password: The password 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 accepts the username
and password or fails when the server rejects either.  On a
successful login, it returns the server\'s response minus the
status indicator.'

def _plaintext(self, username, password):

    return self.user(username).addCallback((lambda r: self.password(password)))
