feat(cordovaEvents): $ionicPlatform.on method

Create $ionicPlatform.on(type, callback) to make it easier to add
Cordova event listeners. Closes #2219

js/angular/service/platform.js

@@ -177,6 +177,28 @@ IonicModule
           return ionic.Platform.is(type);
         },
 
+        /**
+         * @ngdoc method
+         * @name $ionicPlatform#on
+         * @description
+         * Add Cordova event listeners, such as `pause`, `resume`, `volumedownbutton`, `batterylow`,
+         * `offline`, etc. More information about available event types can be found in
+         * [Cordova's event documentation](https://cordova.apache.org/docs/en/edge/cordova_events_events.md.html#Events).
+         * @param {string} type Cordova [event type](https://cordova.apache.org/docs/en/edge/cordova_events_events.md.html#Events).
+         * @param {function} callback Called when the Cordova event is fired.
+         * @returns {function} Returns a deregistration function to remove the event listener.
+         */
+        on: function(type, cb) {
+          ionic.Platform.ready(function(){
+            document.addEventListener(type, cb, false);
+          });
+          return function() {
+            ionic.Platform.ready(function(){
+              document.removeEventListener(type, cb);
+            });
+          };
+        },
+
         /**
          * @ngdoc method
          * @name $ionicPlatform#ready