feat($animate): use promises instead of callbacks for animations

The $animate service (both the service inside of ng and ngAnimate) now
makes use of promises instead of callback functions.

BREAKING CHANGE

Both the API for the cancallation method and the done callback for
$animate animations is different. Instead of using a callback function
for each of the $animate animation methods, a promise is used instead.

```js
//before
$animate.enter(element, container, null, callbackFn);

//after
$animate.enter(element, container).then(callbackFn);
```

The animation can now be cancelled via `$animate.cancel(promise)`.

```js
//before
var cancelFn = $animate.enter(element, container);
cancelFn(); //cancels the animation

//after
var promise = $animate.enter(element, container);
$animate.cancel(promise); //cancels the animation
```