Added implementation for ht_find()
Renamed function: avl_add() -> avl_insert()
Merge pull request #162 from matthiasbeyer/fix-insertion_func_naming

Renamed function: avl_add() -> avl_insert()
Merge pull request #160 from matthiasbeyer/implement-ht_find

Added implementation for ht_find()
Added implementation for ht_insert()