cdn video downloads, seperate folder for each week...

1. CDN video downloads
    a. By default, try to download video from CDN first. If no link found, then use youtube-dl instead. (Youtube is blocked in some countries like China:(, so I make this a first option. And most of courses host video on CDN.)
    b. Use "-nc" or "--no-cdn" to disable this.
   

2. Create seperate folder for each week
    Many courses have too many videos in a module. It's better not to put all files into one folder. Directory structure(course "GG101x The Science of Happiness") now looks like this,
.
├── How to Navigate this Course
│   ├── 01-Intro to edX Video.mp4
│   ├── 01-Intro to edX Video.srt
│   ├── 02-Courseware Video.mp4
│   ├── 02-Courseware Video.srt
│   ├── 03-Navigation.mp4
│   ├── 03-Navigation.srt
│   ├── 04-Discussion.mp4
│   ├── 04-Discussion.srt
│   ├── 05-Summary.mp4
│   └── 05-Summary.srt
├── Week 10 WrapUp
│   └── 01-Video Chat with Emiliana Simon-Thomas and Dacher Keltner.mp4
├── Week 1 Introduction to the Science of Happiness
│   ├── 01-What Makes You Happy.mp4
│   ├── 01-What Makes You Happy.srt
│   ├── 02-Welcome from the Instructors.mp4
│   ├── 02-Welcome from the Instructors.srt
│   ├── 03-Check-Ins Explained.mp4

3. Extracting filenames from <div class="..xmodule_VideoModule"> <h2></h2></div>
    h2 element gives names that make more sense. Vidoes on youtube have names like edX-FA12-cware-4_100(https://courses.edx.org/courses/course-v1:BerkeleyX+GG101x-2+1T2015/courseware/1ff96c6155eb40c39140c656cdc2708b/fe61eec71862458eb2526dbace127fb4/),  RICADBIOT314-V004400_100(https://courses.edx.org/courses/RiceX/AdvBIO.2x/2014T3/courseware/87f77ac155d749bcbb4a41bf1327502e/e56fccd2e69f4d3bb9dd4df929034624/).   


Other changes:

4. Catch exception when using youtube-dl
   If youtube-dl isn't installed, pass the exception and continue.

5.  Extract video section from div(class=xmodule_VideoModule) which make it easier to get CDN download links.