Authenticate a client using it's secret

        :param client_id: the client / service ID
        :param secret: the client secret
        :returns: a Service instance

def authenticate(cls, client_id, secret):
        """
        Authenticate a client using it's secret

        :param client_id: the client / service ID
        :param secret: the client secret
        :returns: a Service instance
        """
        result = yield views.oauth_client.get(key=[secret, client_id])
        if not result['rows']:
            raise Return()

        service = yield Service.get(client_id)
        raise Return(service)