This adds two scripts to be used together with Git over SSH:

* git-auth.py is supposed to be used as AuthorizedKeysCommand. It checks
  whether the public key belongs to any AUR user and invokes
  git-serve.py, passing the name of the corresponding user as a command
  line argument, if any.

* git-serve.py is a wrapper around git-shell(1) that checks whether the
  user passed as command line argument has access to the Git repository
  that a push operation writes to.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>