Pimpl for PhysicsWorld. Provides cleaner API and more effortless usage (one doesn't need to start including Bullet headers when simply wanting to do a simple PhysicsWorld::Raycast f.ex.).
VSConfig.cmd: Use "deps-%VS_VER%-%TARGET_ARCH%" as the deps dir for all build configurations. Use VS2010 as the default generator if no generator passed (re: #668).