implemented getWinner method

src/main/java/ch/zhaw/pm2/racetrack/Game.java

@@ -121,7 +121,12 @@ public class Game implements GameSpecification {
      */
     @Override
     public int getWinner() {
-        // TODO: implementation
+        List<Car> cars = track.getCars();
+        for (Car car: cars) {
+            if(car.getWinPoints() == 1){
+                return car.getID();
+            }
+        }
         return NO_WINNER;
     }