Adding support to list repos

GET /repos now returns a list of repositories. Filtered based on
folder that end in .git and whose repo names are valid.
Checkout the `users` branch: https://github.com/deian/nodegit-http/tree/users
I modified to code to only print the error when in debug more and return a generic error message otherwise.
While we can treat this as part of the "TCB" for now, I think I can move it out of the TCB pretty soon.