Incorporate stripped down and lightly integrated code of Mail::Gpg

We experienced unpleasent surprises (e.g. #502 (closed)) and patched the code anyway, so we now chose to incorporate it (it's licensed under "MIT", which allows that without any obligations). The code is stripped from the parts we don't use, changed in those parts we previously patched (Mail::Gpg#encrypt(), Mail::Gpg::SignPart#verify_signature()), and a little integrated (code of MessagePatch pulled into our own Mail::Message).

Beyond that, the code is verbatim from mail-gpg version 0.4.2 (which is licensed under "MIT").

I don't want to integrate it further because we might want to get rid of gpg in a future moment.