Various fixes for loading embedded records.

This fixes the following issues:
- https://github.com/emberjs/data/issues/540
- https://github.com/emberjs/data/pull/523
- https://github.com/emberjs/data/issues/504
- https://github.com/emberjs/data/issues/516

(I realize there's a lot of work going on embedded records on another branch, but this enables us to use the current functionality with the current master..)