Clean up AVL unit-tests, add content to README.

data_structures/avl_tree/README.md
	-Add an overview of the project's contents.
	-Document `make` targets.
	-Add code demonstrating example usage of the `AVLTree` module.

data_structures/avl_tree/src/test.c
	-Remove `_printData()`, which was used purely for manual
	testing.