Whoops, did all this without committing.
Adjust physics constants.
Some more physics tweaking
Set Platform#shape#object to fix ground detection
Basic player controls.  Physics needs auto force clearing.
More physics tweaks
Run multiple physics steps in one frame.
Make infinite rectangle truly infinite again.
Add camera constraints.  They need tweaking.
Forgot to add object/boundary.rb
Shift dark level boundary a little
Fix direction setting in player
Merge branch 'new-engine' into master

Conflicts:
	levels/dark.yml
	lib/constants.rb
	lib/engine.rb
	objects/player.rb