Update demo page

Hi, I updated the demo page to be a little cleaner.  Most of this happened while I was preparing to integrate jsdifflib into GLSL Sandbox.

In other news, check out GLSL Sandbox Diff, powered by jsdifflib but with a "dark" color scheme applied:

http://glsl.heroku.com/diff#1834.0-vs-1840.1

This pull request contains a white demo page of course.