Fix attribute name typo
Add docstrings
Add __slots__ to Entity

Save a ton of memory from attribute dicts
Delete enemy bullet on collision
Add docstring to engine.py
Remove unneeded __init__
Make Master properties raise NotImplementedError
Add docstring to Master
Change log message
Add hook feature to StateMachine
Fix leftover reference in state.py
Fix docstring textwidth
Add selector resource
Add menu.py
Change main_menu.py
Add exit hook in engine.py
Add exit hook in main_menu.py