Additional Accessors for Node

Nicolò, I'm sending along a few accessors I've found handy using other database-driven tree implementations.  This also adds a test for `getNeighbors` and a **small** change to the `getNeighbors` implementation to reset returned array indexes to start with zero.  Check the commit-message for more information about that change!

I've got 3-4 more of these to do tonight and I'm going to start focusing on handy methods for manipulating the tree.  These were just low-hanging fruit! :smile: 

I'll also update repository docs with these additions.

Hope all is well.