Restructured addon/lib/reference.js

This should resolve #29 and is part of the larger #21 task.

Split `addon/lib/reference.js` into several scripts and moved to new subfolder - `addon/lib/reference/*`

* `list-reference.js` - Placeholder for new `CollaborativeList`, named in consistence with existing constructors. Currently holds `isCollaborativeList` and `serializeList` moved and renamed to `ListReference.isFor˙and `ListReference.serialize` respectively, in order to keep up with consistency.
* `map-reference.js` - Moved from `reference.js`. Holds the `MapReference` constructor.
* `null-reference.js` - Moved from `reference.js`. Holds the `NullReference` constructor.
* `get.js` - Holds a single `get` method, which is the only private method from the old `reference.js` that was actually used in more than one place. Ideally, we won't need this and can get rid of the script completely, but I'm not sure yet.