Added functionality to clean email log table. Cleaning can be enabled/disabled
as well as after how many days entries get deleted (minimum lifetime). The job is triggered
by the event log_log_clean_after, which is dispatched by the Mage_Log module after cleaning
Magento core log tables. This normally happens during cron execution. So cronjobs must be
correctly configured to clean our table regularly. Another option is to manually run the
script shell/log.php.