feat(ngAnimate): ensure JS animations recognize $animateCss directly

JS Animations now recognize the response object returned from a call to
`$animateCss`. We can now setup our JS animation code to fully rely on
$animateCss to take charge without having to call the doneFn callback on
our own.

```js
// before
.animation('.my-css-animation', function($animateCss) {
  return {
    enter: function(element, doneFn) {
      var animator = $animateCss(element, {
        event: 'enter',
        structural: true,
        from: { background: 'red' },
        to: { background: 'blue' }
      });
      animator.start().done(doneFn);
    }
  };
});

// now
.animation('.my-css-animation', function($animateCss) {
  return {
    enter: function(element) {
      return $animateCss(element, {
        event: 'enter',
        structural: true,
        from: { background: 'red' },
        to: { background: 'blue' }
      });
    }
  };
});
```