add(tarsnap): Script to rotate backups

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
Merge pull request #60 from bcachet/tarsnap

add(tarsnap): Script to rotate backups