Add HELO setting in CiviMail config

As related to this forum post:
http://forum.civicrm.org/index.php/topic,33055.0.html

There are other forum posts in the past that have touched on similar issues; I am thinking this would be possible to fix by adding a field to the SMTP mailing settings for the SMTP "localhost" settings (though probably better to call it the HELO settings, because otherwise it's a confusing name).

Then change CRM_Core_Config::getMailer code to load that setting as the "localhost" parameter for the PEAR Mail object if it exists, or default to the existing formulation of $_SERVER['SERVER_NAME'] if it's blank.

As more cloud-hosting options come to be good solutions for Civi, this issue will likely continue to present itself. I believe this approach would take care of multiple situations.