Init missing field values in existing mails. Test metadata upgrade step.
Use default value adapters to fill metadata on mail.
Fix Zope DateTime to python datetime conversion (timezone problem)
Remove specific mail index methods (document_date, document_author, receipt_date, sortable_author).
Move title and message into the common fieldset / Move title at first position.
Revert "Use default value adapters to fill metadata on mail."

This reverts commit d6b4718dbd272d0d28581bbaded8920aa06d9a60.

Conflicts:
	opengever/mail/mail.py
	opengever/mail/tests/test_mail_metadata.py
Make a lower case query to get the author by email (See http://stackoverflow.com/a/16573690/1599111).