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...
```
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);
$animate.cancel(promise); //cancels the animation
```
fix($animate): use $timeout to handle the delay within staggering animations

When transition-delay and animation-delay were used to drive the staggering
animation the result was unpredictable at times due to the browser not being
able to register the generated delay styles in time. This caused a hard to
track down bug that didn't have a solid solution when styles were being used.

This fix ensures that stagger delays are handled by the $timeout service.

Closes #7228
Closes #7547
Closes #8297
Closes #8547

BREAKING CHANGE

If any stagger code consisted of having BOTH transition staggers and delay staggers
together then that will not work the same way. Angular will now instead choose
the highest stagger delay value and set the timeout to wait for that before
applying the active CSS class.
fix(ngModel): treat undefined parse responses as parse errors

With this commit, ngModel will now handle parsing first and the validation
when dealing with the the view value. If any parser along the way returns
`undefined` then ngModel will break the chain of parsing and register a
a parser error represented by the type of input that is being collected
(e.g. number, date, datetime, url, etc...). If a parser fails for a standard
text input field then an error of `parse` will be placed on `model.$error`.

BREAKING CHANGE

Any parser code from before that returned an `undefined` value
(or nothing at all) will now cause a parser failure. When this occurs
none of the validators present in `$validators` will run until the parser
error is gone.
feat(ngModel): provide validation API functions for sync and async validations

This commit introduces a 2nd validation queue called `$asyncValidators`. Each time a value
is processed by the validation pipeline, if all synchronous `$validators` succeed, the value
is then passed through the `$asyncValidators` validation queue. These validators should return
a promise. Rejection of a validation promise indicates a failed validation.