Fix right-click for Firefox and IE

Fix #310

src/utils/Browser.js

@@ -0,0 +1,17 @@
+/**
+ * xterm.js: xterm, in the browser
+ * Copyright (c) 2016, SourceLair Private Company <www.sourcelair.com> (MIT License)
+ */
+
+/**
+ * Browser utilities module. This module contains attributes and methods to help with
+ * identifying the current browser and platform.
+ * @module xterm/utils/Browser
+ */
+
+
+let userAgent = navigator.userAgent;
+let platform = navigator.platform;
+
+export let isFirefox = !!~userAgent.indexOf('Firefox');
+export let isMSIE = !!~userAgent.indexOf('MSIE');