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-client.js

@@ -1,5 +1,4 @@
 // for non background pages
-/* global chrome, Components, safari */
 
 (function() {
 'use strict';