RADIUS module update. (#627)

* The authenticator field is now a XStrFixedLenField (this improves its readability when displayed using show() or show2()).

* RADIUS module update.

* The description of the compute_message_authenticator() method (RadiusAttr_Message_Authenticator class) has been edited.

* The "Response Authenticator computation" test requires the Cryptography library.

* Fix attribute names in compute_message_authenticator().

* Applied guedou's comments.

Specify field length in the Message-Authenticator attribute.