Support for setting up logging method

Added support for setting up logging method which will be used in case of fails and errors (with unittest).

While using smoketest together with monitis (online service for services' monitoring), I got some alerts from monitis (meaning that the our smoketest's response didn't contain word 'PASS').
As there is no way to find out what the response was at that point (at least I don't know how to do it within monitis), I needed a way to see error and fail messages even in case when anybody else loads smoketest which fails.
So, I added a parameter in constructor of SmokeTest class which will be the logging method (it should be able to receive string message as parameter, and log it somewhere).