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.
So, we got a response on SO, but it is basically the same approach that I am currently using: http://stackoverflow.com/questions/24461964/method-for-disabling-a-button-in-dat-gui

I want to move this forward, but I also want to make sure that it meets your needs. So, we have a couple of choices it seems:
* Remove the code for disabling/enabling buttons and use some other method to prevent the function from being called when we don't want it called.
* Keep the method that we have, perhaps finding a way to do it more elegantly.

Let me know what you would prefer.