Make the screen a rectangle.

Modify the ball to collide with "collidables".  Add an interior
rectangle so the ball bounces inside.

The interior rectangle is a hack. What we need to do is setup the system
with planes that have normals.
Refactoring in Block/Interior Rectangle

I'm extracting a Plane object out of the Rectangle, and giving the
Rectangles better domain object names, the Block and World. Both of
those will inherit from Rectangle.
Delete unused code
Add a world object.

Got the "InteriorRectangle" in as "World". Now it's under test as well.
We should be able to make blocks for the actual game now.