+ Implemented timestamps
* renamed `VideoInfoItem` to `VideoPreviewInfo`
* Moved streaming service-related classes into their own, new package: "services"
+ Added javadoc to some classes and methods (where functionality is known well enough to explain)
- De-duplicated common fields between `VideoInfo` and `VideoPreviewInfo` by moving them into a common superclass: `AbstractVideoInfo`
- Removed 2 methods in `PlayVideoActivity` which only call `super()`, and therefore are unnecessary: `onResume()` and `onPostCreate(Bundle)`
+ Added `VideoInfo(AbstractVideoInfo)` constructor
- to support converting `VideoPreviewInfo`s into `VideoInfo`s, to reuse scraped info (yet to be implemented)
* 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, such as fetching `Jsoup.Document`, and `playerArgs:JSONObject`
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