added physics module
gravity works
abstract modules from app.js into individual files
Merge pull request #13 from salarkhan/tweak-physics

Tweak physics
alter how inertia builds, implement friction, refactor movemente code
Merge pull request #14 from salarkhan/tweak-physics

alter how inertia builds, implement friction, refactor movemente code
Merge branch 'master' into playing-field

Conflicts:
	app.js
	index.html