AnimationCollection cleanup

`AnimationCollection` refactor (now `Tween` and `TweenCollection`) for #1188.  Tweens are used throughout Cesium for camera flights, camera bounce in 2D, scene morph, and the selector widget.

As some of us discussed, I suggest we mark this `@private` since it has a ways to go before it is the "CSS Animations for Cesium properties" it was originally envisioned to be.  Some ideas are in the new roadmap (#1830).  I encourage someone to take this over post 1.0 so we have really nice mouse over effects and so on.

Please start looking at this.  There are just a few small things left:

* [x] Mark `@private` (and update CHANGES.md)
* [x] Remove Sandcastle examples
* [x] Add tests
Should this be documented as `Tween~CancelCallback`?  (and obviously that type would need documentation as well).  Otherwise there's no information as to the signature of this function.