First Commit of Animation

Here it is.

I'm feeling pretty good about the API but the real test will be when someone other than me ports to it or builds their own bot with it. 

If anybody is willing to try this out I will bend over backwards to help them. I need the perspective of someone who is new to it.

I've got some basic documentation over in docs/animation.md but it's not really written well for beginners. Once the API is golden I need to write a wiki page that's more of a tutorial. There is a lot to digest here.

This PR is the culmination of all the discussion in #304 also fixes #362
Is `tween.progress` always an integer (lazy, yes I know)? If so, then use `!Number.isInteger(tween.progress)`; if not, then use `!Number.isFinite(tween.progress)`, instead of `isNaN`. If guarding against division by zero, there result will be `Infinity` or `-Infinity`, in which case `isNaN` is no help: 

```js
var n = 1 / 0;
n; // Infinity
!Number.isInteger(n); // true, negation satisfies this condition and enter the consequent
!Number.isFinite(n); // true, negation satisfies this condition and enter the consequent
isNaN(n); // false, this would proceed and the next function would pass `Infinity` to the easing function!
```