feat($animate): complete refactor of internal animation code

All of ngAnimate has been rewritten to make the internals of the
animation code more flexible, reuseable and performant.

BREAKING CHANGE: JavaSript and CSS animations can no longer be run in
parallel. With earlier versions of ngAnimate, both CSS and JS animations
would be run together when multiple animations were detected. This
feature has now been removed, however, the same effect, with even more
possibilities, can be achieved by injecting `$animateCss` into a
JavaScript-defined animation and creating custom CSS-based animations
from there. Read the ngAnimate docs for more info.

BREAKING CHANGE: The function params for `$animate.enabled()` when an
element is used are now flipped. This fix allows the function to act as
a getter when a single element param is provided.

```js
// < 1.4
$animate.enabled(false, element);

// 1.4+
$animate.enabled(element, false);
```

BREAKING CHANGE: In addition to disabling the children of the element,
`$animate.enabled(element, false)` will now also disable animations on
the element itself.

BREAKING CHANGE: Animation-related callbacks are now fired on
`$animate.on` instead of directly being on the element.

```js
// < 1.4
element.on('$animate:before', function(e, data) {
  if (data.event === 'enter') { ... }
});
element.off('$animate:before', fn);

// 1.4+
$animate.on(element, 'enter', function(data) {
  //...
});
$animate.off(element, 'enter', fn);
```

BREAKING CHANGE: There is no need to call `$scope.$apply` or
`$scope.$digest` inside of a animation promise callback anymore
since the promise is resolved within a digest automatically (but a
digest is not run unless the promise is chained).

```js
// < 1.4
$animate.enter(element).then(function() {
  $scope.$apply(function() {
    $scope.explode = true;
  });
});

// 1.4+
$animate.enter(element).then(function() {
  $scope.explode = true;
});
```

BREAKING CHANGE: When an enter, leave or move animation is triggered then it
will always end any pending or active parent class based animations
(animations triggered via ngClass) in order to ensure that any CSS
styles are resolved in time.