'Get the L{IResource} avatar for the given credentials.
@return: A L{Deferred} which will be called back with an L{IResource}
avatar or which will errback if authentication fails.'

def _login(self, credentials):

    d = self._portal.login(credentials, None, IResource)
    d.addCallbacks(self._loginSucceeded, self._loginFailed)
    return d
