AVLTree changes and others: SEE description

Added a public access method to AVLTree to get height of AVLNodes.
Changed many public methods to private. Changed a testing condition in
verifyAVL(). Overall I think AVLTree could use some debugging. I
commented up some things that might point out some bugs.

I couldn't think straight enough to start figuring out how test cases
would work for TestAVLTree but did find some examples here -
http://stackoverflow.com/questions/3955680/how-to-check-if-my-avl-tree-implementation-is-correct

I finished commenting all our classes. So that's done, woohoo.