Publish gst-python on PyPI

The common way to pull in dependencies for a Python project is to install them via pip: either directly, or via an install_requires stanza in setup.py. This requires the dependency to be available on PyPI. As gst-python is not available on PyPI, it can't be pulled in in the normal way.

I realise that the package is somewhat special, requiring various GStreamer headers and introspection data be installed. But if PyGObject can do it, presumably gst-python can too.