Expose crypto info for a given email message

Having the mail (LeapMessage) interface exposing the relevant crypto info for a given message would make much sense in terms of having a unified entry point to the mail services. However, this should just expose the underlying methods in keymanager.

First we should agree on the API design, and then move to implement the keymanager api.

(from redmine: created on 2014-11-12, relates #6355 (closed), relates #6914 (closed), precedes #6366 (closed))