initial pass at libgit2 page

I've completed a first pass at the libgit2 landing page.

The Usage Guide button still points to http://libgit2.github.com/api.html - are we going to host all of those docs on the git-scm.com site as well?

Also, be sure to check the new page out on a retina MBP. I made all of the graphics 2x and they look pretty awesome on that display. I'm planning on going back and updating the git-scm graphical assets as well.

Let me know if you have any changes and how'd you'd like to go from here - mainly in regards to the Usage Guide.

cc @schacon @tanoku