code: function (customEvent, callback, filter, prepend) {
          return Event.onceBefore(customEvent, callback, this, filter, prepend);
      }
docs: Subscribes to a customEvent on behalf of the object who calls this method.
  The callback will be executed `before` the defaultFn.
  The subscriber will be automaticly removed once the callback executed the first time.
  No need to `detach()` (unless you want to undescribe before the first event)
  
  @method onceBefore
  @param customEvent {String|Array} the custom-event (or Array of events) to subscribe to. CustomEvents should
  have the syntax: `emitterName:eventName`. Wildcard `*` may be used for both `emitterName` as well as `eventName`.
  If `emitterName` is not defined, `UI` is assumed.
  @param callback {Function} subscriber:will be invoked when the event occurs. An `eventobject` will be passed
  as its only argument.
  @param [filter] {String|Function} to filter the event.
  Use a String if you want to filter DOM-events by a `selector`
  Use a function if you want to filter by any other means. If the function returns a trully value, the
  subscriber gets invoked. The function gets the `eventobject` as its only argument and the context is
  the subscriber.
  @param [prepend=false] {Boolean} whether the subscriber should be the first in the list of before-subscribers.
  @return {Object} handler with a `detach()`-method which can be used to detach the subscriber
  @since 0.0.1