Checks if the exact version of Tarsnap is already installed and, if so,
skips the download and build steps.

Data loss will occur if backing up postgresql while it runs. A simple
fix is to stop it during the backup. I've moved the backup to early
morning to reduce possible downtime.

A better approach would be to use pg_dump.

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