Use emptied event to reset player states

[heff]

In #1124 we added a feature to let the player reset its 'has-started' state when a new source is loaded. It used the loadstart event to do this.

In #1271 we found that the play and loadstart events could fire in any order, so we made a fix so that the the firstplay event would be fired even if the play event came before the loadstart.

In #1271 we also realized that emptied is the event we should be relying on to reset the player states. It always comes before the play event, so it can greatly simply the code we currently have for the firstplay event.

However, the Flash SWF doesn't fire the emptied event, so before we can use it we have to first implement it in Flash. Adding a related issue on the swf repo. videojs/video-js-swf#100

[gkatsev]

Just ran into this. With autoplay and ads on safari, we end up getting the play event before loadstart but ads even up pausing the player before loadstart fires so we think we switched videos and we reset the started state but we shouldn't have because we didn't actually change videos.