Remove debug statement
Update documentation
Merge separate engines into one

This change allows us to freely arrange all systems and cuts
down on the number of virtual function calls (although in a very
minor way).
Remove debug drawing system

We can do this more effectively now that Bullet and Ogre run in
the same thread.
Remove SharedState templates

We don't need those anymore. Maybe we can bring it back later
for sharing data between threads.
Update EntityManager doc and rename update to processRemovals

The name makes the intention much clearer
Move EntityManager and LuaState into Engine

I don't like making the Engine class even bigger, but those two
just belong there.
Update docs and remove some unused typedefs