added parameter for pingpong playback

This adds the option for pingpong style playback.  Can be called by adding 'pingPong: true' to the parameter array when calling load_animation.

I've also included a demo of pingpong playback - the effect doesn't work well on the existing demo footage due to the static background - artifacts appear during reverse playback (see below image - not sure if there's a way to prevent this?).  The pingpong effect works best when the whole screen changes for each frame - ie. video footage rather than animation.

![artifacts](https://f.cloud.github.com/assets/1153616/1868302/f79f3aba-7866-11e3-956f-cb27466891fd.png)

The footage used in my demo is royalty free from [movietools.info](http://www.movietools.info/video-background-loops/sport-loops/item/loop-196-basketball-loop.html)