* added VideoInfo(AbstractVideoInfo) constructor, to support later implementation for reusing info scraped into VideoPreviewInfo, into VideoInfo
* Made the Extractor class behave as a per-video object;
- most method return values are video-specific, so it makes sense (to me) to have Extractor be stateful.
- The only stateless methods are getVideoUrl(), getVideoId() and loadDecryptionCode(String)
* Implemented a constructor for YoutubeExtractor, which performs all initialisation work
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