Factor out mail Parser

In the current code in the mail 0.3.x branch, the mail parsing happens right into the addMessage method. We need to factor out the message parsing, and use the generic Parser in the future IncomingMail module. From the processing of the incoming mail on, a generic Message object (or some kind of message wrapper, if we do not want to mess with the soledad store adaptor until the docs have not been saved) should be passed around, instead of the raw string.

This should expose a clean point for a generic hook where other components can register: keymanager, for instance, will need to check attachements looking for signatures, etc.

(from redmine: created on 2014-11-12, relates #6361 (closed))