code: function createCar(car) {
      var back = createCardBack(assets.carBack);
      var front = new CarFront(car);
      var flippable = new FlippableCard(back, front);
      flippable.x = flippable.regX;
      flippable.y = flippable.regY;
      return flippable;
  }
docs: car: Optional reference to car. If omitted, creates a blank car