Update edx-dl.py

1. According to edX's new feature shk3/edx-downloader#73 , download subtile and video from button link directly. This should resolve shk3/edx-downloader#98. The modification is based on shk3/edx-downloader@e3b67cc9038905141b362c1675595c973f4db7e1.
2. Save video and subtile with filename like "1.3 - Human Thought: Mind vs. Body, Reason vs. Emotion, Cold vs. Hot Cognition.mp4"。
3. Resume downloading video from CDN.

Note: If we download files from CDN directly, we needn't rely on youtube-dl anymore.