Proof of concept

ping @Ocramius @danizord

First try for #132 .

This PR does not work, it's just a quick prototype. As said in the issue, this assumes that we have an hydrator that only extract scalar names, and let associations untouched (so extract will actually return associations, BUT as object).

It's then the ResourceRenderer that recursively calls the hydrators.

I suspect this to be quite slow but... it seems the "best" way to do it no?

TODO:

* tests
* check circular rendering
* allow to override association directly in ResourceModel.