Change root from Entity to StateTreeNode

[prdoyle]

The root node of a bosk state tree has always been an Entity, with an ID. We've never had concrete plans for this; it has always been more of a gut feeling that we wanted it to be an entity. There were some early ideas about using the ID to identify different versions of the bosk state so you could switch between them, or even have multiple versions live at the same time in some way. But none of these ideas materialized.

Experience has shown that picking an ID for the root object is always a rather silly exercise, because you know it will never be used for anything.

We should probably change the type declaration of Bosk to: public class Bosk<R extends StateTreeNode>.

[prdoyle]

Done in #52