Add "Fork me on Github" ribbon

I add a ribbon to the top right corner. I also set `z-index: 101;` for it because `.header` is `z-index: 100;` – we don't want the ribbon to be shyly hidden behind anything. ;)

And this is where I got the ribbon from:
https://github.com/blog/273-github-ribbons

There are other colors! ;)