Add CSS transition support

Is it obvious what [`Ember.copy`'s](http://emberjs.com/api/#method_copy) second argument does?

```javascript
Ember.copy(obj, true);
```

Or the first optional arg to [`jQuery.extend`](http://api.jquery.com/jQuery.extend/)?

```javascript
$.extend(true, obj1, obj2);
```

The answer is no, but what *is* obvious is that there is something going on, and you need to investigate further. Still, it's not necessarily a very persuasive argument...