Introduce drupal_alter for commerce_buckaroo_parsed_result

This depends on #1992870: Various small fixes, #1992878: Move functions to include files and #1992880: Push functionality being applied in that order.

I inserted a drupal_alter at the end of commerce_buckaroo_result_parse(), so that you can e.g. change the encoding of an order ID in the 'BPE_Invoice' parameter if you want different invoice numbers, or implement extra checks to see if a Buckaroo Push request is valid. Examples are in the .api.php.

There are also comment changes in here. I'll separate those out in a separate patch on request only.