Node references

Allow properties to directly reference nodes. Previously, properties couldn't contain a node reference in a way that was meaningful outside of `model/javascript` or that would survive replication.

This is a predecessor to `branch/collection-events`, which needs to send a node reference in `node.children.added` and `node.children.removed`.

@eric79, please review. @allisoncorey, @scottnc27603, @jessmartin, you also may be interested.

On second review, I don't see how to do this. The explanation is too long to fit in the first sentence without diluting the description. It really needs to follow in the second. I added [#3799](http://redmine.virtualworldframework.com/issues/3799) to define the concept explicitly in the API documentation.