Remove unused intersections test code
Remove particles test file
Move physics demo to main index page
Rework physics demo a bit
Shift key for slow time
Extract World and Display into their own files
scale parameter for world constructor
Extract main entry point into its own file
Default restitution of 0.3
Extract Rectangle into its own file
Handle physics when only one body is present
Rework polygonal body initialization for super() calls

Including density in lieu of mass
Switch to density for initialization. It's all relative anyway.
Add asteroid generator
Test many asteroids colliding