A script to backup all you public github repositories. This is designed to be
run from cron, e.g. like this:

    0 0 * * * ~/bin/backup-github.py <username> <directory>

For every one of <username>s github repos, a bare repo will be created under
<directory> that tracks all remote branches.

If you don't redirect output to /dev/null, you'll get mail (via cron) whenever
a new repo was created or a new branch is being tracked.