* @param string $category The category of usage to fire a trigger for. A full list of categories can be found in the `Usage Categories documentation <http://www.twilio.com/docs/api/rest/usage-records#usage-categories>`_.
* @param string $value Fire the trigger when usage crosses this value.
* @param string $url The URL to request when the trigger fires.
* @param array $params Optional parameters for this trigger. A full list of parameters can be found in the `Usage Trigger documentation <http://www.twilio.com/docs/api/rest/usage-triggers#list-post-optional-parameters>`_.
* @return Services_Twilio_Rest_UsageTrigger The created trigger