Update README.md replacing Collusion for Lightbeam

The only place I didn't feel like to change was on the license part sayin: "The Original Code is collusion."

I can change that too if you want.
renaming all image files from collusion_* to lightbeam_*
renaming all icon files from collusion_* to lightbeam_*
Replacing Collusion to Lightbeam
Merge branch 'master' of https://github.com/mozilla/lightbeam into name-replacing

Conflicts:
	data/content-script.js
	lib/ui.js