Use unittest.main() method to execute tests

'unittest.main' is simpler and more generic than creating a custom
TestSuite and running a TestRunner on it. Also it automatically takes
care to return the correct exit status.
The previous change made this "bug" visible
The models created by modelConverter did not have
the 'real' attribute.

That made model_locator fail
Merge pull request #37 from pymad/fix-test-exit-status

Tests now exit correctly in case of failure