WIP: Add a juju restore subcommand.

This is the CLI portion for the work we are doing on cleaning up the juju-restore plugin (and turning it into a normal command).  For now it just stubs out the actual call to the restore client.
I'm fine with it either way, but I don't have a good sense of the general API design yet to have a reliable opinion. :)  I'll check with Nate, et al.