Revert "Physics entities can now use multiple fixtures"

This reverts commit 79bc2e8ed1bd78514fa326ab0c8fb5385ee12b40.
Physics entities should now be able to have mulitple fixtures

This fixes bug #79. Physics entities should now be able to have multiple
fixtues each with it's own polygon shape. Each polygon is still limited
to 8 verticies as Box2D suggests. The commenting is so you can see
what's going, obviously it needs editing.