Compress Postgres backup file

fixes #156 by adding the -L flag, as suggested by @ventolin

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.

Remove all configuration for MySQL and configure PostgreSQL as the main
database.

All *_mysql_* options have been changed to *_db_* options.

Postgres requires the database user to have a password in order to
connect via localhost. The db_admin_password option is used to set the
password of the admin user (usually postgres).

(No more "Removing leading '/' from member names" warnings)

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