Feature/verify signatures

As promised, a first version of verifying PGP signed messages. It’s not completely finish, yet, but it works as a starting point. I still need to debug some issues. (Had problems verifying some signatures, msg.verifySignature() always returned false :\)

Basically, what I did is follow your hints from #150. I also change the background-color of the wrapper that contains the plain-text message according to the verification state (yellow should be for unknown/untrusted keys, green for trusted keys).

Any comments welcome :) I will start sorting out the remaining issues and use this PR to keep you posted.