Linked data

@jrolfs 

- ```link: true``` will cause updates to the collection to update the models ids array.  Successive ```get```s with ```link:true``` will return the same collection.
- ```link: false``` or ```link: undefined``` will result in normal brainstem behavior (no singleton-ish collection, no updating ids array)
This still needs to use array syntax: `@_associatedCollections[field]` and failing specs need to be added for this current behavior.