Fix #37 nonstop calculations.

The updateBallPhysics would do a if(ballHitPaddle(...)) whenever
the ball coordinates were > paddleLevel. But paddleLevel is
paddle.getCoordinates.getX() and the second paddle happend to have
coordinates.setX(0). So _everything_ was causing the calculations and
overwhelming the CPU.

Refactored some related code in the ball physics path.