Update look and feel

This is a visual update of the site, to make it thematically match with http://libgit2.github.com. I hacked together a new logo (note the ".NET™-blue" brick on the bottom), and changed the background color.

![libgit2sharp 2x](https://uploads.github.com/raw/ben/libgit2sharp.github.io/stylize/images/libgit2sharp@2x.png)

- [x] Final logo
- [x] Typography