Fix incorrect parse of url in video_module.py

Sometimes the video url may look like 'http://abc.com/path/video.mp4?xxxx'.
(For example, the Windows Azure's media service will offer such type of url.)
The original code in video_module.py will produce 'mp4?xxxx' instead of 'mp4'
as the extension of a filename for such type of url, and therefore the video
will be non-playable. The fix here uses the built-in urlparse module to retrive
only the path component from a url and therefore the extension 'mp4' will be
correctly fetched.