Fixed CMakeLists for moved file
Removed comOffset property

No idea why the Bullet developers deigned to name it comOffset.
It doesn't have much to do with the center of mass, it's the
transformation between physics and graphics that's applied in
the btDefaultMotionState. We don't use that anymore, so we don't
need the comOffset property.
Added a header to CMake source list, just in case
Added Keyboard global to Lua scripts

You can now query the keyboard state from Lua by using
Keyboard::isKeyDown
Switched rigid body movement from force to impulse