Added support for programmatically skipping to a specific frame.  Fixed issue with reversing playback.

A feature add and a few bug fixes:
* Add support for display of a specific frame, with the format .spriteanim('frame',n) to display frame n
* Fixed multiple calls to 'play' creating more than one executing loop, and resolved issue that FPS was not applied immediately; both as the timer object was never set.
* Make animations stop immediately, rather on next frame iteration - resolves issue with needing to call play twice twice after calling 'stop' twice.