Rewrite opensubtitles api integration using the json api

I rewrote OpenSubtitles integration but it is not complete. I rewrote OROpenSubtitleDownloader class but other classes need to update.

Known issues are:

• The fetched languages are not localized
• Subtitle metadata isn't showing correctly on tableviewcell
• The language picker shows language code instead of the full name

Also this is my first objective-c contribution so I am open to feedbacks.

Related issue: #1702 (closed)