Implemented avl_destroy()
Merge pull request #65 from matthiasbeyer/implement-avl_destroy

Implemented avl_destroy()
Added implementation for avl_add() with helper
Added implementation for helper: new_avl_el()
Added helper function: insert_element_into_tree()
Added implementation for helper func: element_with_hash()
Added implementation for inserting an element into the tree
Added functions (without bodies) for rotations