Add "References" as well as the "In-Reply-To" header

See http://www.jwz.org/doc/threading.html for details. Some co-workers
of mine have a mail client that doesn't properly thread without
this. Since we're only referencing one message its value can just be
the same as "In-Reply-To".
Merge pull request #21 from avar/add-references-to-headers

Add "References" as well as the "In-Reply-To" header