Add a juju subcommand for backups.

This will fit on top of the client API that Michael is writing.
Like Michael said, this just factors out the default filename from his changeset.  I figured we could add the template into his backup API code.  One advantage is that I was able to use the template in the help message, so it should always stay in sync.  I was just imagining what would make things easier if at some point we switched to bz2, etc.