Show python version when running tests
Make compatible with Python 3.3

So VimSwitch is now compatible with Python 2.7 and Python 3.3

Other changes:
- Some small code cleanups
- Made the FileSystemTestCase safe operations slightly more generic. They now pass extra arguments as *args and **kwargs. So there is no need to explicitly specify argument names and their defaults. It also helps because some arguments have changed between python versions.