Document the options parameter of $animate functions

Unless I missed it, there is no documentation explaining the `options` parameter that is optional in a bunch of the `$animate` service's functions. Ex:

``` js
$animate.enter(element, parent, [after], [options]);
```

I just spent a bit digging through the source to finally find you can pass in an object like this:

``` js
{
  addClass: 'some-new-class',
  removeClass: ' some-old-class'
}
```
