Add docstrings and make pep8 changes

This PR brings pep8 fixes and docstrings. I have also removed the definition of `TextureGroup` from the main module and instead imported it from `pyglet`.

I love this project and I think it has a great potential to inspire kids who want to get into programming. To that end I think documentation and pep8 styling will make this easier.

If I get more time in the future I may go through and add more comments explaining the actual implementation. (I need to learn pyglet first ;))