Add page templates that can be included automatically in themes that don't even declare support for Eventbrite.

This allows a user to install the plugin and get events in a reasonable presentation within their theme, even if the theme has done nothing to explicitly support the plugin. If support is declared and templates are specified, then the plugin defers to the theme.
Correct page template name for themes using the plugin's templates.
Correct setting of is_single for event single views.
Add the event name slug to its URL for single views.
Docblock, inline commenting, and whitespace improvements.
Rename is_eventbrite_event() to eventbrite_is_event() for consistent prefixing.
Make Eventbrite_Rewrite::add_query_vars() publicly accessible.