Ember serializer

Needed this for a project I'm working on. It has a Laravel backend with an Ember.js client that relies heavily on including documents. It was ridiculously easy to make, just copied JsonApiSerializer and modified a single line. I've not run into problems for my project with this one-line-changed adaption, and I couldn't imagine no one else would be waiting for this, so I decided to add some tests and write some documentation.

Let me know what you think!