Add eventable to json response on events
Fix incorrect url response

Some of the urls were incorrect and this should be html_url not
app_url because that code preceeded app_url
Add remaining todos endpoint docs
Add documentation for calendar event subscribers.
Fix json response vars in Events

The json payload was incorrect for Events API

Original issue: https://github.com/basecamp/bcx-api/issues/76
Merge branch 'api-add-docs-for-calendar-event-subscribers'
Merge branch 'update-json-response-on-events'
Merge branch 'add-active-todos-to-api-docs'