Renamed the HandshakeChallenge to HandshakeChallenger and HandshakeResponse to HandshakeResponder.
Renaming of a member.
Improved javadoc.
Added the capability of error codes so that a code can be sent encrypted to the other side.
Removed the error code, since it's not possible to decrypt the message since the server side does the most time not have the client sides Public Key.
Changed the visibility of two constants since they are also needed in a subclass.
Added tests and fixed one bug, where the handshake was not destroyed after an error occurred.
Adapted to the changes in beam-commons MessageField/ContentField.