Fix the url processing error when there is a query string inside the video source's url

1. In video_module.py, rewrite the get_ext() function to use the built-in urlparse module to parse the input filename first and then get the file's extension name from the parsed path.
2. In 02_html5_video.js, first check whether the url has already had a query string, and if so, then use '&' instead of '?' to prevent breaking the url's integrity.
3. In test_video.py, add two test cases (one with query string while the other without) in order to test the rewritten get_ext() function.