Added unit tests for tbd avl() balance method for BST.
Refactored bst.balance() a little bit to accomodate TBD avl().
Added _rotate_left and_rotate_right methods, and unit tests for those.
Updated BST.insert() to support AVL balancing when given a balance=True arg
Updated BST.delete() to take a balance arg and, if it's true, run _avl() on each ancestor of the deleted node to ensure that the BST remains balanced.
Removed old test method for test_bst.py that wasn't running anyway.
Updated BST.__init__() to also accept a balnce arg, and build a balanced tree with an incoming list of value. Also added one more unit test for that, and cleaned up some other unit tests.