feat(Animation): Animation for AngularDart.

Final pull request for NgAnimate. This is prefered to the original long running https://github.com/angular/angular.dart/pull/551 version.

Demo is located under `demo/animate_demo/web/index.html` and currently includes:
* `ng-if`, `ng-select` and `ng-show`\`ng-hide` examples.
* `ng-repeat` examples.
* `ng-class` examples.

Animation library currently includes:
* `window.requestAnimationFrame` based `AnimationLoop` implementation instead of timer delay for the animation loop.
* Integration with `ng-if`, `ng-unless`, `ng-switch`, `ng-show`, `ng-hide`, `ng-repeat`, `ng-cloak` and all of the `ng-form` implementations.