added notification_email attribute
auto-include rdiff-backup and chef config in backup
added autotrim capabilities
fixed directory listing
added sudo capabilities

The rdiff-backup user has to be able to backup files belonging to the root user.
For this purpose, capabilities of running `rdiff-backup --server
--restrict-read-only /` via sudo have been added.
the server now runs rdiff-backup::client to backup chef and rdiff-backup files
client sorting in backup.sh and ssh_known_hosts
removed unused `var_dir` attribute
don’t add a mail address to the cron job if it does not exist
updated README
set standard attributes for initial release