generic subtitle download.
The idea is that all subtitle downloaders must descend from SubtitlesIE
and implement only three basic methods to achieve the complete subtitle
download functionality. This will allow to reduce the code in YoutubeIE
once it is rewritten.
Now is using the same length as the flash player used for age protected videos, but the algorithm is different, so now for age protected videos it first tries to use the old algo.
Allow adding download tests right in the IE file.
This will cut down on merge conflicts and make it more likely that new IE authors will add tests right away.