New Tester and Bug Fixes

These changes add a new Tester class, which allows for representation of
Entity objects in three dimensions. The way I have it set up will draw
every Entity object that has been currently loaded. Using this, I was
able to find and fix several bugs that I had completely overlooked.
These bugs affected the gravitational pull of Entities, causing the
velocity changes  to occur improperly. This has since been corrected.