Clear wait_interval on restart

In response to the reply here https://github.com/defunctzombie/zuul/issues/97#issuecomment-65977739 and the issue highlighted in https://github.com/defunctzombie/zuul/issues/97#issuecomment-65977621. Clears the interval so that waiting statements aren't printed after a browser is restarted. It's an ugly solution, since I'm attaching the interval to the browser, but listening for the restart event with zuul. It might be nicer to eventually have `restart` emitted by the browser itself, rather than have zuul emit browsers through both `browser` and `restart`.