Fixed the MEU bug of the Enigma format.

The Enigma machine (rotors I-II-III with initial position M-E-U)
encrypts the message “AAAA” to “GDXT”, not to “XDTT”. Fixed the
interaction between rotor notches and rotor positions.
Added M3 rotors to the Enigma format.

Added descriptions to multiple choice options. Added an addChoice
method to MultipleChoiceOption that allows manipulating choices after
the option has been initialised.