Use urlparse.urljoin() instead of str.join()

`urlparse.urljoin()` takes care of a couple of things which may be missed if `str.join()` is used.

Please have a look at [documentation of `urlparse.urljoin()`](http://docs.python.org/2/library/urlparse.html#urlparse.urljoin) and it's [implementation](http://hg.python.org/cpython/file/026ee0057e2d/Lib/urlparse.py#l250).