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.

This differs from { embedded: 'always' } in that the record itself is not saved with its parent, only the ID reference.

So I wrote an alternate embedded functionality: { embedded: 'ids' }.

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