Refactored 2-body simulator (elliptical orbits)

I extracted a "fall" block to make the original code much DRYer.  No more duplicated code for each sprite.  I also made mass and velocity local to each sprite.
This make it possible to have as many sprites as you want, each gravitaionally attracted to all the others.
I'm going to play with that next.  Maybe even simulate the whole solar system.