Changed to global config, moved some stuff out into main module

- Dropped that nifty ConfigParser extension that I did before in favour
of a pure Python config module that can just be imported into any other
module.

- Moved initialization of stuff into the startup module for, well,
modularity.
Create new Tileset class

Abstract out tileset operations to reduce graphics calls inside cell and
sprite classes.
Refactor level data and window view

- Now one tiled map object per level, which is passed directly to the
game

- Screens now store a reference to the window's view (i.e. surface)
rather than to the entire window object
Refactor level success/failure

These checks belong in the level class, not in the game or player class