Add contribution for integration with gitolite

Add in contrib/gitolite/patches-g2/ a patch to apply
to gitolite v2.x source such that gitolite can delegate
the read/update of some repositories to git-mirror proxy.

Add support for local-base configuration that can be used to
specify the prefix used for local repositories. This prefix will
be removed if the proxy forwards the request to the master server.
Increase minor version to v0.3