re-do the backup script to use the duplicity program

Duplicity will manage the process of creating incremental backups for us.
Although duplicity can both encrypt & copy files to a remote host, I really
don't like PGP and so I don't want to use that.

Instead, we'll back up to a local directory unencrypted, then manually
encrypt the full & incremental backup files. Synchronizing the encrypted
backup directory to a remote host is a TODO.