Add Model.source for backend envelopes

In certain situations, the model and its nested attributes are each served in their own envelope.

For example:

```
{
  'User': {
    id: 1,
    name: 'John'
  }
  'Document': {
    id: 35,
    title: 'Doc title',
    content: 'Doc content'
  }
}
```
This pull request adds `Model.source` that is used to return any remaining attributes after the associations have parsed them.

Simply, if `source` is provided and it's in the attributes, `resp.source` will be returned. Otherwise, just `resp` will be returned.