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 is probably where you want to specify the multiple association behavior. Probably be referencing the collection from the `Model#_associatedCollections` object instead of the local variable. Currently it will always be `model._associatedCollections.field` which is incorrect and should fail. Once `_linkCollection` is fixed to use array notation the collection should be available as a property of `_associatedCollections` named after the field.