Add cron dump support for databases other than MySQL.

Use tasks.py's own dump_db command to dump the database, instead of using
mysqldump directly.

Always overwrite the /etc/cron.daily script with a new one, in case the
contents have changed (e.g. switch to using dump_db).
Merge pull request #87 from aptivate/fix_sqlite_scheduled_database_dumps

Add cron dump support for databases other than MySQL.