Support embedded objects for DS.ActiveModelSerializer.serialize

This is a compatible way for [embedded
objects](http://emberjs.com/guides/models/defining-models/#toc_embedded-objects)
using `DS.ActiveModelSerializer`.

In 0.13:

``` javascript
DS.RESTAdapter.map('App.Post', {
  comments: { embedded: 'always' }
});
```

Using this patch:

``` javascript
App.PostSerializer = DS.ActiveModelSerializer.extend({
  attrs: {
    comments: { embedded: 'always' }
  }
});
```