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.

## ToDo

* [x] add decent mail-signed-96.png (or may be even smaller, see next point)
* [x] re-design the initial frame indicating a signature
* [x] work around openpgp problems with plaintext and signatures
* [x] handle unknown public keys
* [x] don’t crash if openpgp.read_message returns null/undefined
* [x] add/test popup version of verification frame
* [x] refactoring: hide openpgpjs implementation details in pgg model class
* [ ] make everything beautiful (ui-wise)