Introduce message types for WBE protocol

* Abstract message class added - all messages types are
  derived from it now and have to implement the `to_dict`
  method, so it can be serialized and transferred with
  broker;
* Implemented the `Response` message type, that restores
  failures from dictionary on creation;
* Corrected and improved unit tests;

Change-Id: I10e017a613f0422420d0244b9f8786f988863107
Merge "Introduce message types for WBE protocol"