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.
The buttons aren't actually disabled - meaning the "onclick" binding to the function still exists. The method that I used is that I put a `<div>` inside the button that grays it out, making it look disabled. So, in order to detect whether it is disabled or not, we check for the existence of this `<div>`. Since the `<div>` is actually inside the button, the click is actually registered, which is why we have to capture the click within the function, and decide there whether to execute it or not.

I tried to add the HTML attribute, "disabled" to the DOM elements of the button, without success.

Other options might be:
1. Remove the onclick function binding from the button when it is disabled and save this somewhere so it can be added back when the button is enabled.
2. Have two divs for each button. One - the functional div, and one a disabled-looking dummy div. When the button is enabled, show the functional div, and when the button is disabled, show the dummy div.
3. Figure out a way to position a div over the button instead of inside it, so it can't actually be clicked. But, this would be tough to figure out the positioning.

Do you happen to know of a straightforward way to disable the buttons other than the way I am currently doing it?