Merge pull request #26 from SergioBenitez/gh-pages

Added a live code editor based on Ace.js and rust-playpen.
Draw tab guides every two column instead of four
Merge pull request #27 from SimonSapin/patch-1

Draw tab guides every two column instead of four
Merge branch 'gh-pages' of github.com:graydon/rust-www into gh-pages