Create AVL tree exercise

[tboychuk]

AVL is a balanced Binary Search Tree (BST).

Create and implement new exercise in AVL Tree:

• create new branch exercise/{ticket_number} from master
• create module called avl-tree.
• implement exercise and create pull request from exercise/{ticket_number} to master
• create new branch exercise/completed-{ticket_number} from exercise/completed
• merge changes from exercise/{ticket_number} into exercise/completed-{ticket_number}.
• provide completed implementation and create pull request from exercise/completed-{ticket_number} to exercise/completed

Please note that that you should create two pull requests. The exercise, and the complete implementation.

Use Binary Search Tree as an example.