Reorganized files, refactored code, began menu implementation

The code is currently split into three main files: main.py, snake.py,
and mainmenu.py.  Others will be added for other menu options.

The code has been significantly cleaned up.  Snake.py is now a class
as opposed to a group of global functions.  Works much better and
can be easily run from the menu.

A countdown has been added before game start.