Support embedded objects extraction

I supported embedded objects extraction via `DS.ActiveModelSerializer`.

When the related objects are embedded in parent object such as the following json:

``` json
{
  "post": {
    "id": 1,
    "comments": [
      {"id": 1, "title": "hi", "body": "Foo"}
    ]
  }
}
```

We can extract it via its serializer:

``` javascript
App.PostSerializer = DS.ActiveModelSerializer.extend({
  attrs: {
    comments: { embedded: 'always' } // or 'load'
  }
});
```