feat($animate): coalesce concurrent class-based animations within a digest loop

All class-based animation methods (addClass, removeClass and setClass) on $animate
are now processed after the next digest occurs. This fix prevents any sequencing
errors from occuring from excessive calls to $animate.addClass, $animate.remoteClass
or $animate.setClass.

BREAKING CHANGE

$animate.addClass, $animate.removeClass and $animate.setClass will no longer start the animation
right after being called in the directive code. The animation will only commence once a digest
has passed. This means that all animation-related testing code requires an extra digest to kick
off the animation.

```js
//before this fix
$animate.addClass(element, 'super');
expect(element).toHaveClass('super');

//now
$animate.addClass(element, 'super');
$rootScope.$digest();
expect(element).toHaveClass('super');
```

$animate will also tally the amount of times classes are added and removed and only animate
the left over classes once the digest kicks in. This means that for any directive code that
adds and removes the same CSS class on the same element then this may result in no animation
being triggered at all.

```js
$animate.addClass(element, 'klass');
$animate.removeClass(element, 'klass');

$rootScope.$digest();

//nothing happens...
```