git-http-backend

I've been critical of the server-side design of git in the past and thought I could design a better one... but honestly the simplicity of the CGI design has grown on me. I think the easiest way to get `isomorphic-git` into the server is to just straight up implement `git-http-backend`. Let it go ahead and parse URLs - instead of a `dir/gitdir` argument like normal we'd have a `gitProjectRoot` dir and combine it with the path. We can break `git-http-backend` down into individual `git-upload-pack` and `git-receive-pack` services and expose more granular functionality later if needed.