Convert RCTContextExecutor to use a dispatch queue instead of a thread #75

This is a mostly straightforward task but may need some changes from Facebook's side if there is existing code calling into -[initWithJavaScriptThread:globalContextRef:]. The only tricky thing I see is the check in dealloc to see if the current thread is the JS thread. That could be replaced with dispatch_queue_set_specific and dispatch_get_specific, or dealloc could simply always dispatch_async. I'm also not sure it's needed at all since JSC should be thread-safe.