Scripts used to rotate tarsnap backups exit when a command fails
Cron will catch that return code is not zero and will send message to root

No more silent failure

Add tasks to install script/tarsnap configuration file and a cronjob to run
the backup job
Number of hourly/daily/weekly/monthly backups kept can be configure from the
tarsnap.sh script

Thanks to this script, we don't use all the backups which save space on
tarsnap server and thus reduce the price to pay

Refs #39