Introduce mock of sdbus library through extracting its interface. Set up unit tests of Connection class through injection of sdbusMock to constructor. Clients of Connections class should use fabrics instead.
* Add preliminary changes for async server methods
* Refactor the Message concept and break it into distinctive types
* Continue working on async server methods (high-level API mainly)
* Continue developing support for async server
* Finishing async server methods
* Finishing async server methods (fixing tests & cleaning up)
* A little code cleaning
* Add unit tests for type traits of free functions
* Support for generating async server methods in stub headers
* Update ChangeLog for v0.3.0
* Update the tutorial with how to use async server-side methods
* Update the TOC in sdbus-c++ tutorial
* Update numbering in TOC
* Remove unnecessary code
* Final cleanups
