[RFC] swiftmailer support

Hi,

I'd like to hear your opinion about adding a Swift_Transport implementation to Stampie as per this PR (not complete yet, see below). This would allow Stampie to be used as a drop-in package for every project already using SwiftMailer.

If the idea is basically fine with you, I'd like to discuss the implementation of Stampie\Integration\SwiftMailerTransport::getStampieMessage(). This method is needed as long as the \Swift_Mime_Message implementation in question does not also implement Stampie\MessageInterface (not likely if we want to use the Transport transparently in other projects).

To be able to convert between the messages, getStampieMessage() will require a version of Stampie\MessageInterface with the setters defined as well.

I'm not quite clear why you left out the from and subject fields/accessors from Stampie\Message, otherwise this could be used as well for conversion?

Cheers