Separate $.one event handler.

src/js/lib/event.js

@@ -0,0 +1,11 @@
+/* Copyright (c) 2015 Hyunje Alex Jun and other contributors
+ * Licensed under the MIT License
+ */
+'use strict';
+
+exports.once = function (element, eventName, handler) {
+  element.addEventListener(eventName, function (e) {
+    element.removeEventListener(eventName, handler, false);
+    handler(e);
+  }, false);
+};