[ticket/11488] Use correct base class in email notification method

In ticket/11451 this was not correctly changed to reflect the new class
phpbb_notifcation_method_messenger_base. Additionally, an undefined
variable error has been fixed in messenger_base.php (change should be
confirmed by bantu though).
The copyright year was changed according to Nils' remark in the previous PR #1300.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11488

PHPBB3-11488