git repositories

We now have the ability to clone a Buzzy repository from a remote git
repository.  Right now this functionality is only available in the C
API; next step is to add it to the URL and YAML repository handlers so
that you can clone a git/Buzzy repo via a repository link.
Load git repositories from URLs

You can now specify a "git://" or "git+foo://" URL in your buzzy
repository's `links.yaml` file.  That will cause buzzy to clone a copy
of the remote git repository, and use it as a linked buzzy repository.
!git repository links

If you use a bare `git://` URL in your links.yaml file, you'll get the
master branch of the remote repository.  If you want some other branch,
you should use a YAML mapping, specifying the `url` and `commit`
directly.  You must use the `!git` YAML tag to tell buzzy that the
mapping describes a git repository.
Merge branch 'feature/git-repo' into develop

* feature/git-repo:
  !git repository links
  Load git repositories from URLs
  git repositories