code: def checkin(checkin_id, options={})
        response = connection.get do |req|
          req.url "checkins/#{checkin_id}", options
        end
        return_error_or_body(response, response.body.response.checkin)
      end
docs: Retrive information about a single checkin.
  
   @param [String] checkin_id the ID of the checkin.
   @param [Hash] options
   @option options String :signature - Signature allowing users to bypass the friends-only access check on checkins