Added an overview of the game system design.

This is a pretty rough estimate of the final system, but it's a good
start.

I've tried to design it so that any sub-system (Application,
RenderingEngine, PhysicsEngine, Interpreter) can be replaced without
affecting the others. The API layers between OpenGL on the bottom and
the window mangers at the top act as failsafe layers. In the case of the
window manager we can abstract platform specific functionality. The
OpenGL abstraction is only there to deal with new OpenGL versions.

The only big thing not in the chart are platform specific libraries for
misc. functionality like multi-threading and clocks. I think every
sub-system will require a library like that, so just imagine a big
static library that everything uses.

Brain: If you sync it should put you back on the master branch. You've
been commiting to master, so it must be something on your end.