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 needs to use array `[]` syntax in order to leverage dynamic key names. As it stands you will only ever have `{ collectionName: ... }` and it will get clobbered if you have more than one linked collection association. Multiple associated collections should also get a spec (eg: a `Post` with `GoogleDocument`s and `Attachment`s).