Add transactionResponse.responseCode checks

[mdubbs]

These updates to the PaymentTransactions examples should help to alleviate some of the confusion as to what constitutes a successful transaction (not just API response) coming back from Authorize.Net when using the authorizenet gem.

The current examples don't show checking the actual response code to verify if the transaction was successful, rather they show only the check on whether the Authorize.Net API successfully processed the request (not if the transaction was successful, declined, held for AFDS, failed for some other reason).