Add support for multipart/related messages

When inlining an attachment (typically an image for use in an HTML email), the MIME type multipart/related is used, as opposed to the multipart/alternative MIME type typically expected with both an HTML and text body.

See:
http://en.wikipedia.org/wiki/MIME#Related
http://tools.ietf.org/html/rfc2387

It seems there's no branch for 2-1-stable as the readme would imply, so this is against master.