[WIP] Makes JSONAPISource (more) JSON API spec compliant.

Sending WIP to discuss over code. Review very much appreciated!

Spec: http://jsonapi.org/format

### To do

- [ ] Fix `update` tests
- [ ] Fix integration tests
I don't like the name of the variable, or the syntax, to put the `serialized` object into the `type` namespace. Hopefully there's something better.