Undefined property: Notification::$mail

I'm not 100% sure this is a bug, but I'm getting the following error from my crons.

> PHP Notice:  Undefined property: Notification::$mail in /var/www/MPOS/public/include/classes/notification.class.php on line 158

> PHP Fatal error:  Call to a member function getError() on a non-object in /var/www/MPOS/public/include/classes/notification.class.php on line 158

It appears the `Notification` class extends the `Mail` class instead of `Base`, in which case `$this->mail` does not exist.
Merge pull request #1050 from headzoo/patch-1

Undefined property: Notification::$mail