Make the youtube plugin work again.

Still not able to play videos with encrypted signatures.
Set video-channels scraper api in stone and centrilize some repetitive task.

Also added youtube-dl executable to the package. THANKS!
Great youtube scraper improvements. Can now play every video on youtube, thanks to youtube-dl

* use the new scraper v4 api
* call youtube-dl instead of calculate the real url !! :)
Clean imports in onlinevideo
More explicit import in plugins __init__.py