Fixes numenta/nupic.cerebro2#35

Addresses issue of allowing player to go beyond number of actual iterations. Also introduces code to automatically enable/disable player buttons depending on the state of the animation.
I don't know of a way, but I've started a bounty on [your StackOverflow question](http://stackoverflow.com/questions/24461964/method-for-disabling-a-button-in-dat-gui) so hopefully we'll get a good answer from that.