Add --self-test option

Users can run our unit tests only before installation, when they have a source code available. It could be beneficial to allow them to run our tests anytime, to check if Pythoscope is working correctly. Pygame folks implemented this as a library function (http://renesd.blogspot.com/2008/09/pygametest-moving-testing-forward.html), but since Pythoscope is more of a tool than a library, I think something like a --self-test command line option would be more appropriate.