YoutubePlaylistIE: try to extract the url of the entries from the media$group dictionary

Extracting it from content can return rtsp urls.
YoutubeIE: If not subtitles language is given default to English for automatic captions (related #901)
YoutubePlaylistIE: don't look into entry['content']['src'], accruing to the docs this can return live stream urls
Merge pull request #888 from rg3/youtube_playlists_fix_886

YoutubePlaylistIE: try to extract the url of the entries from the media$group dictionary (closes #886)