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.
This method (the old `serializeList`) is never actually used, as far as I can tell. The reason for that is how the `MapReference.prototype.set` is implemented.