Upgrade reveal.js

The previous version wasn't supporting side notes for presentations
not named index.html.

* reveal.js is now cloned in a submodule
Fix font and background
Add Capybara slides
Bump reveal.js
Merge branch 'gh-pages'

Conflicts:
	2012-12-20-gemnasium.html
	2012-12-20-redis.html
	2013-01-24-capybara.html
	index.html
	reveal.js
Merge branch 'master' into gh-pages