Add Gemfile, adapt Rakefile, ...

Yes! The site finally looks right for me now! The last thing that did the trick btw was finding the proper 2.x-Version of bootstrap-sass.

I've also pdated the contributing-page to reflect the proper (as in "works for me" :wink:) way to run the preview, I've added a description for myself and changed the order of the listed contributors to "chronologic". Just didn't feel right for me to be listed on top of Gunnar and Sjaak...

@sjaakd, please check if you can build the site now as well - and if so, add a sentence or two about yourself as well, if you like. There's enough room :wink: