Port hook_ec_paymentapi() implementation to new hook_ec_transaction_[op]()

The hook_ec_paymentapi() is gone. See this issue for details #506966: Remove $op from hook_ec_transactionapi().

The new hook_ec_transaction_[op]() hooks are documented here:

http://cvs.drupal.org/viewvc.py/drupal/contributions/modules/ecommerce/e...