Subtitles rework + support for dailymotion

Hi,
I just heavily refactored the subtitle support. In this moment is based in a big class called SubtitlesIE which makes most of the logic of the subtitle options, then for each website you must implement only one method to download the subtitles.
Please check the patch and help integrate it with yours.
I push this to your repo since I so your pull request in the main repo, and I think this could be a nice evolution towards the goal of a nice subtitle support.