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.

A _value_ is a property value, method parameter or result, or an event parameter--basically any JSON-representable value that is involved in the simulation state. Some values may now be references to nodes. These look like `{ id: *node-id* }`, but are blessed in such a way that no application-created value may be mistaken for a node reference.

This function determines if the value provided as the parameter is a blessed node reference or not.