Toggle visibility of diffs, fixes #64

Heya, I added some simple js, so now you can click on the commit header on the page and toggle the visibility of all files in the current commit view. Not sure if that was what you were thinking of, but it could be useful for having a quick glance over all changed files in the commit. I also did some slight changes to the CSS and removed some extra border.
Bit of warning though, I'm kinda new to Git and Github so please tell me if I'm screwing up something.