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://f.cloud.github.com/assets/39902/1550209/38075d28-4dfe-11e3-939c-dd7d8de8af2c.png)

- [ ] Final logo
- [ ] Typography