AnimationStateDelegate

addresses #31 
I think it's a good idea to split it up since prevSheet() is designed to be called when the "previous stuntsheet" button is clicked on the html page. I think it's more user-friendly to have the "previous stuntsheet" take you to beat 0 of the previous stuntsheet instead of the last beat. If you disagree, I can change it. Otherwise, if you think it will improve clarity, I can replace the call to this.prevSheet() with this._currSheet--