* 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
8a0b972c78