embedding IDs only and tests

Hi Ember Data!

I'm using MongoDB with Ember Data, and didn't find a way to specify hasMany relationships where lists of object IDs are embedded in a parent object, and the child objects are sideloaded on get, but the child objects have no reference to the parent.

Not sure if this is in line with your development plans on ember-data or not, but I needed this for my datastore and branching Ember Data was the easiest way to implement. So here it is -- if this is useful, I'm happy to follow up with more tests / more tightly specifying behavior / etc.

Cheers,
Gabe