Add API_KEY management to Chiketto

- added #api_key and #api_key= for reading and writing an Eventbrite API
  Key, if a key is not set Chiketto will fall back to the
  EVENTBRITE_API_KEY environment variable.
Add start of Attendee and update Resource (Version++)

- Added new attendee object with basic attrs, no costs, barcodes, or
  profile yet.
- Updated Resource to allow blank objects
- Updated tests for API_key to fix issue in other tests