Support the event_search endpoint to power a new default query (only public live events).
Simplify supported query parameters for Eventbrite_Query. This eliminates the need for complicated processing of query params before the API call, and (along with the now separate endpoints for public vs. private events) will greatly simplify pagination math still to come.

* With separate endpoints handling public and private events, the display_private filter is no longer needed.
* Removed support for 'order' and 'orderby'. We'll stick to the API defaults for now.
* Removed support for 'status', and made 'live' the default status for user_owned_events (rather than 'all', which includes past events).
Tweak a docblock description.
Correct results returned by Eventbrite_Event::get_instance().
Correct missing parameter in Eventbrite_Manager::do_event_search() and Eventbrite_Manager::get_user_owned_events().
Add the missing 'public' property from event_search results. The API doesn't return the property because all queries on that endpoint are for public events.
Removed filter which is no longer used.
Correct markup in the event listings template.
Modify Eventbrite_Event mapping.

* Corrected post_date to the event's start time.
* Added the GMT time.
* Renamed 'organizer' to 'organizer_name' (since we also have 'organizer_id').