Allow the Scour module to be used as a standalone program

Created by: Ede123

This PR allows to use the Scour module as a "standalone program" (e.g. download, unzip and run) without the need to install the module itself and/or any third party modules.

It consists of two parts:

• Use a local copy of "six". We should not expect six to be available in the users Python environment. E.g. Inkscape comes with a minimal installation of Python that does neither include six nor allows the user to easily install it.
• Add a simple wrapper script around the Scour module that can be executed directly. The relative imports introduced in d1c66cc7 have broken the previous approach to achieve this with if __name__ == '__main__': run()