refactored whole project based on a sample project from Apple

camera needs to be enhanced (some features are still in TDMap which is not used anymore)
AI could be enhanced too, more specifically the one for the spawn points
find optimization for path finding (some kind of cache?)
implement collisions using the physics engine
add some buttons on the HUD?
add some buttons on the debug layer?

eventually, get rid of TDMap and TDGameScene