Encrypt/decrypt can also sign/verify.

Also Implement code review comments for openpgp sign.

* Add assertions and exceptions to openpgp encrypt/decrypt/sign/verify methods.
* Added comments where they will help.
* Make code more clear by encapsulating more the access to GPG wrapper and
  removing concatenation of ascii armored keys.
* Add verification of fingerprint of verifying key.
* Shorten check for signing key id.
Add changes file.
Merge remote-tracking branch 'drebs/feature/2488-openpgp-sign-3' into develop