Add support for $mail->addReplyTo()

Currently there's no support for PHPMailer's method for setting the ReplyTo header.

This is pretty useful for a lot of cases where the email comes "from" the site-information address (so as not to be seen as spam), but contains an end-user's address in the ReplyTo, allowing them to actually reply to the author of the message directly in email.

Basically we'd need to extend the function that invokes the PHPMailer class to look for a Reply To in the $params['headers'] and invoke its addReplyTo() method.

I'll see if I can add a patch along these lines shortly.