add a way to whitelist attributes to serialize.

I needed to pick which nested attributes are being send to the server.
Just add an array of key to the relation.

```coffeescript
    relations: [                                                                                                                                            
      type: 'many'
      key: 'thumbnails'
      relatedModel: -> Entities.Thumbnail
      serialize_keys: ['id', 'x', 'y', 'height', 'width']
    ]
```