Expose signature and encryption information on the public API

As a result of a discussion with the pixelated team: "Check feasibility of signature and encryption information exposed on the public API for leap mail 0.4.1 (raise priority for this, we should plan to brainstorm on additions to API together with meskio)"

(from redmine: created on 2015-04-29, relates #5993 (closed), relates #6363 (closed))