Add support for channel9 (Implements #1885)

There is wide diversity of possible links formats at channel9.msdn.com and moreover, sometime it is impossible to guess between video link and playlist. For example:
1. playlist: http://channel9.msdn.com/Events/GoingNative/2013 
2. video: http://channel9.msdn.com/Events/GoingNative/2013/Cpp-Seasoning
3. playlist: http://channel9.msdn.com/Events/TechEd/Australia/2013
4. video: http://channel9.msdn.com/Events/TechEd/Australia/2013/KOS002
Here, both **video 2** and **playlist 3** has four segments in path.
So, I've ended up with single extractor for all kind of links that determines link type according to page content.

There are also sometimes links to slides and zips that are worth to be downloaded as well. Not sure about youtube-dl's policy on non-video files, but for now I've just masked them as video files to get downloaded.