Duplicity integration for encrypted backups to S3

Initially I attempted to use the AES encryption and S3 support available in backup_migrate, but when testing the restore process I found that it was extremely slow with a large database. So slow in fact as to be unusable.

Because I needed a short-term fix I opted to backup via a shell script using duplicity (http://duplicity.nongnu.org/), but I think a better long term solution would be to integrate duplicity support directly into backup_migrate. If there is interest in this I would be willing to work on the integration, most likely as a sub module.

Duplicity is a command line utility that uses the rsync library and gnupg to create encrypted incremental backups that require limited bandwidth and disk space. My testing reveals that it is *many* times faster than the current AES encryption. Because of the set-up required to use duplicity it would not be feasible to use it in a shared hosting environment and it would require technical knowledge on the end-user's part in order to create the GPG keys and install duplicity.