feat(doubletap): add onDoubleTap directive

Closes #2292

Author: adamdbradley

js/angular/directive/gesture.js

@@ -1,4 +1,4 @@
-var GESTURE_DIRECTIVES = 'onHold onTap onTouch onRelease onDrag onDragUp onDragRight onDragDown onDragLeft onSwipe onSwipeUp onSwipeRight onSwipeDown onSwipeLeft'.split(' ');
+var GESTURE_DIRECTIVES = 'onHold onTap onDoubleTap onTouch onRelease onDrag onDragUp onDragRight onDragDown onDragLeft onSwipe onSwipeUp onSwipeRight onSwipeDown onSwipeLeft'.split(' ');
 
 GESTURE_DIRECTIVES.forEach(function(name) {
   IonicModule.directive(name, gestureDirective(name));
@@ -38,6 +38,22 @@ GESTURE_DIRECTIVES.forEach(function(name) {
  */
 
 
+/**
+ * @ngdoc directive
+ * @name onDoubleTap
+ * @module ionic
+ * @restrict A
+ *
+ * @description
+ * Double tap touch at a location.
+ *
+ * @usage
+ * ```html
+ * <button on-double-tap="onDoubleTap()" class="button">Test</button>
+ * ```
+ */
+
+
 /**
  * @ngdoc directive
  * @name onTouch