Add conditional cron jobs for db and media backup

Add hourly db backup cron job if settings.PG_BACKUP_DIR
Add nightly rm backups 4 days old if settings.PG_BACKUP_DIR
Add hourly media rsync if settings.MEDIA_BACKUP_DIR
Merge pull request #132 from jgmize/db-media-backups

Add cron jobs for DB and media backups