Allow implementation of additional sending mechanisms by setting Mailer ...

This change allows implementing additional send methods. Eg you set "$ph->Mailer" to be "mymailer" and then you implement the method "mymailerSend". This of course in a derived class.

I'd like to use it to implement AmazonSES, which I did in https://github.com/michield/phplist/blob/master/public_html/lists/admin/class.phplistmailer.php but this change allows for a much cleaner way of extending phpMailer.