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
+1 to @piotr's comment re: standardization and clarity.

On Wed, Oct 15, 2014 at 4:59 PM, Piotr Mitros <notifications@github.com>
wrote:

> In docs/en_us/data/source/internal_data_formats/tracking_logs.rst:
>
> > @@ -600,9 +745,16 @@ of the video file and play automatically stops.
> >     * - Field
> >       - Type
> >       - Details
> > +   * - ``current_time``
> > +     - integer
>
> No strong insight. My preference is floats. 100ms resolution is adequate,
> but there's no reason to not have maximum precision. UTC is good.
>
> What's even better is standardization. In other words, sending out exactly
> what we were doing before. Breaks in format are painful. They lead to
> broken code, incorrect results, etc. depending on pipeline, and you often
> don't find out until many hours in. Legacy or new events in an inconsistent
> format are not good.
>
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