Feature/postrolls

Add support for postrolls.
Depends what you mean by a video completes.  When the main video completes the state will always be "content-ended".  This is a signal to the ad integrator that he has an opportunity to play a post-roll.  When the post-roll completes, it will return the player back into a "content-playback" just like it would after any other ad.  That simplicity in logic seems fine to me, do you have an alternative in mind that you think makes more sense?