t937 work in progress: Added commerce_authnet_fundraiser_commerce_response_fields() callback placeholder, and Authorize.net response fields selection table as a 'tableselect' element.
t937: Refactored the form logic for the mappable response fields table. Now it looks good and leaves a nice, flat array of field names in the $config array.
t937: Added #default_value so that mappable fields checkboxes reflect the current configuration.