[Add] VAD support to detect when speech ends

@blandinw I don't think wrapping the code of WITRecord.start/stop into a dispatch_async(main_queue) is a good idea because it does other things + it makes it "async". I added some comment on the start and stop function so everybody know that they should be called in the main thread. 
An improvement to that would be to call it on a specific thread, but this should be done on another pull request.