Handle alternative YouTube video links

There are several URLs that can lead to YouTube. The video ID can be easily scraped from them though. Several examples, found in the YouTube DL tests

• http://youtu.be/BaW_jenozKc (I believe this is https now, but it shouldn't matter)
• http://www.youtube.com/v/BaW_jenozKc (Also probably https now)
• https://youtube.googleapis.com/v/BaW_jenozKc
• https://www.youtube.com/watch_popup?v=BaW_jenozKc (found this in test_youtube_extract function)