polyfill SpeechRecognition

The vendor prefix will eventually be removed, please use `window.SpeechRecognition` if available instead of strictly depending on `window.webkitSpeechRecognition`.

Also is there a way to update the article at http://updates.html5rocks.com/2013/01/Voice-Driven-Web-Apps-Introduction-to-the-Web-Speech-API ? /cc @gshires