Issue #1618256 by Steven Jones: Added a drush command to drain the mail_queue() without all of cron.

#356200 by aufumy, xamount: use 'administer site configuration' permission, and added instructions on how to know the mail key

a dsm() error.
Removed code duplicated in queue_mail.module.

job_queue module, which sends them via cron. Useful for high traffic sites
that send a lot of emails.