[youtube] move subtitles methods from the base extractor to YoutubeIE
[subtitles] rename SubitlesIE to SubtitlesInfoExtractor

Otherwise it can be automatically detected as a IE ready for use.
[subtitles] Simplify the extraction of subtitles in subclasses and remove NoAutoSubtitlesInfoExtractor

Subclasses just need to call the method extract_subtitles, which will call _extract_subtitles and _request_automatic_caption
Now the default implementation of _request_automatic_caption returns {}.
[subtitles] Use self._download_webpage for extracting the subtitles

It raises ExtractorError for the same exceptions we have to catch.
[howcast] Do not download from http://www.howcast.com/videos/{video_id}

It takes too much to follow the redirection.
Merge branch 'subtitles_rework' (closes PR #1326)
Credit @iemejia
[youtube] Support automatic captions with original language different from English (fixes #1225) and download in multiple languages.
[subtitles] Also list the available automatic captions languages with '--list-sub'
[youtube] Fix detection of videos with automatic captions