Add a juju subcommand for backups.

This will fit on top of the client API that Michael is writing.
Actually that code is effectively already in the state/api file in a PR of mine. Eric has just factored it out here to reuse it. (It's the default filename used by the client Backup method.) An alternative would be for that code to not use a default and have the CLI always pass in a filename. The danger of just hardcoding it in the help message is that we could change the format and then forget to change the help message too. (There isn't a constant in the apiserver code as it doesn't send a filename.)