Add .jshintrc, and use the "use strict" directive

.jshintrc's otion-set is a personal choice, merely a suggestion.
Beside that, it includes some common globals for specific browsers, so
there's no need to set the globals in every .js file.

In order to force strict coding, "use strict" directive was added into
every .js file.

src/js/vapi-common.js

@@ -1,5 +1,4 @@
 // only for background and other extension pages
-/* global chrome, Components, BeforeLoadEvent */
 
 (function() {
 'use strict';