Starterkit: Top-Down Racing Game with GameplayKit

Similar to the racing demo in this article: http://tilemapkit.com/2015/06/combine-gameplaykit-agents-spritekit-physics/

Features:
- AI Cars follow path, with sensible (but not realistic) car behavior
- Keep track of each car's position in the race, ensure cars can't just take shortcuts
- Physics collisions with other cars and shapes
- Different car behaviors/attributes based on terrain (gravel, sand, grass, tarmac)
- Obstacles like jumps or oil slicks (car becomes uncontrollable)
- Multiple Levels with TMX files and example tilesets
- Player controls (left/right, accelerate/brake)