Fix up empty docblocks.
Implement query parameters for API calls.
Update docblocks and clean up some whitespace.
Mark the API query parameter "cancelled" as invalid. Accoring to docs it's supported, but returns an error when used in a call.
Add pagination to events listing.

The Eventbrite API returns events in chunks of 50, with pagination information. This pagination info is used to set WordPress pagination, in a way that only the chunk of 50 that applies to the page will be called. Currently, math is kept to a minimum by working with a fixed 10 events per page (meaning we're not supporting the WP_Query `posts_per_page` argument, nor `Settings > Reading > Blog pages show at most`). This will come in the future.
Remove commented code. We'll stick with the shortest possible prefix for transients.
Add the "eventbrite-event" class to events.