Fix for bug "os.path.join() is used for building urls"

To support Windows `os.path.join()` replaced with `posixpath.join()` in namespaces.py and oauth.py.
I've replaces it with posixpath to make it easier then to adapt it for both python2 and python3 (`urlparse.urljoin` should be used in python2 and `urllib.parse.urljoin` for python 3).