#28 Add support for Vbr-Info header

[Patrick Vande Walle]

This is defined in RFC 5518 :http://www.rfc-editor.org/rfc/rfc5518.txt
That would be particulary useful on outgoing messages. While this header could be added by some MTAs, it seems that the DKIM signing program is the best place to inseert the Vouch By Reference header. See the RFC for more details.

Thanks