Use Mail gem over RMail

Tracking issue.

Move to the Mail gem (https://github.com/mikel/mail) over RMail. Previous and related issues:
- #22 Problematic RMail
- #61 Related pull-request (first attempts)

The main difference from #61 is that the same message id (no safe id / ref) is used so no changes are made to the Index for now. It might even be possible to migrate the old index..