Initial pass at implementing an 'adscanceled' event

I've added an `adscanceled` event to the state machine, allowing us to bail out from `content-set` or `ads-ready` if we don't want to show any ads. 

This is for the case where we've made an asynchronous call out to an ad server, and the ad server has told us that it doesn't have an ad for this particular video. In this case, we'd just trigger `adscanceled`, and that would short-circuit the whole ad playback mechanism, instead of having to wait for the timeout.