provide NO_PHYSICS build option to build without physics (WIP)

This isn't ready for prime time -- probably PhysicsController wants to
be stubbed out some Game.cpp is not so #ifdeftacular, and there are
other places that are a bit ugly too, but this illustrates the reach of
the physics engine.

I took bullet out of the include path, preventing any inclusing of
Physics*.h because the btAlignedAllocator causes link errors if any
Physics* objects that wrap a bt* object are ever instantiated even if
they're never otherwise used.