Mobile events, updates to video events

@stroilova , @mulby , @nasthagiri , @gwprice , @lou-wang , @mhoeber here is a framework that we can work from to get initial documentation out with the release. Notes:
- I am not sure that the same additional context fields that I see for the v.0 play_video (Olga shared with me) event also apply to all of the other video events.
- the event field names for the new edx.app.browser.launched event in Olga's spreadsheet are very rough
@mulby, do you mean, ``path`` is provided for all of the 8 existing video events, regardless of the source of the event (browser vs. mobile)? or do you mean, ``path`` is added to the context field for every event that we emit (it is a "common" field)?