Renamed Cipher since we never use another mode but CBC.
Moved the all Message classes into a separate package as their numbers recently grew.
Renamed MessageField class, the inner enums and partially fields for the sake of less to type and a more 'natural' representation of the message format in this class.
Shortened the names because they are quite long. The "message" in the name was redundant anyway since it already is in the package name and the extended class.
Access some often used static methods and constants now via static import. This results in less gibberish and shorter lines.
Cleanup - not used.
Refactored code using code inspection.