Bouncy ball! 

This includes the other commits of #1, and fixes it's fault. That PR is now deleted. 

Ball bounces between top and bottom. 

Compared to the other pull request:

ball.setVelocity() will change the speed of the ball in real time.

When we drew the ball we drew with uneven pixels. I round the positions and dimensions to whole numbers and draw the ball

When we hit a wall, we do not need to recalculate K. Instead, we just flip either dX or dY (depending on the which wall we hit).