Update aura website

A preview is available here : 
http://hull.github.io/aura/

To update the website
* checkout the `website` branch
* install middleman (`bundle install`)
* do your edits, commit, push
* do `rake publish`

[middleman-gh-pages](https://github.com/neo/middleman-gh-pages) will automatically checkout the `gh-pages` branch, do a clean build, and push to github, and checkout the `website` branch again.