New hook implementation for altering outgoing mail configuration

I've implemented a new hook to be used to alter outgoing email settings. It's called in CRM_Core_Config::getMailer(), just before the Mailer object is initialized.

I'm using the hook for two reasons:

1. Some hosts in our set use the CiviSMTP-recommended Postfix forwarder, but not all of them can be configured that way.
2. It was not possible to do this via hook_civicrm_config, since the config settings for mailing_backend are not accessible via the $config singleton.

I'm using the hook in a 4.1.5 Drupal 7 install, and it works. I'm not yet set up for work against the current CiviCRM trunk, although the patch is against today's 4.3 based trunk. The implementations of CRM_Utils_Hooks and CRM_Core_Config are very similar; I expect that this will work on 4.3, and will not interfere in any way for installations that do not use the hook.

I include documentation in drupal/civitest.module.sample, and would be happy to add documentation to the 4.3 wiki if the hook is accepted.