Set reply-to to customer for shop-owner and supplier mails.

The sender of all mails is always the shop owner, because of the SPF.
When the shop owner (or the supplier) response to the order email and does not change the recipient, the mail will be sent to the shop owner.

In order to make it easier to respond to the customer I've added a `Reply-To`-Header to customer and supplier emails, set to the customer email.

I've only updated the Plone 4 part of the ``MailHostAdapter``, since we no longer officially support older Plone versions and it works differently for Plone 3.