Map E-Mail headers to Document metadata and set defaults

- [x] Init metadata on mail
- [x] Author handling
- [x] delivery_date special handling
- [x] Upgrade step 
- [x] Move title and message into the common fieldset.
- [x] Fix mail indexes. 

Discuss:
- [x] metadata file: metadata behavior has a `archive_file`. I guess it makes no sense to fill this with the same data too. Check definition for OGIP6 `●	Datei: Enthält die *.eml Datei mit der Originalnachricht (ggf. nach Konvertierung `
- [x] references on extracted documents: Do i have to implement anything? imho it's already done. 
Hmm. We currently don't even create `ftw.mail.mail` objects for sent mails. The `IDocumentSent` event is therefore fired for `opengever.document.document` objects :grinning: - so we still need to implement this (creating `ftw.mail.mail` objects for sent outbound mails), and I would suggest to handle this in a different PR.