PayGate: add support for refund

Closes #1486
BraintreeBlue: Expose processor_response_code

This allows us to reveal the underlying Braintree response code to
customers who care about it:

https://www.braintreepayments.com/docs/ruby/reference/processor_responses
Cybersource: Add verify ability

I would have liked to have a remote test or two for verify.
Unfortunately, I can't seem to get the Cybersource remote tests to run.
The credentials I have are working, but I'm getting an error code of 150
(General system failure) in each of the tests.  It's maddening.  I'm not
sure if there's something amiss with our Cybersource sandbox account or
not.  I'm left to wonder if anyone can actually run them.
BraintreeBlue: Improve processor_response_code

The error code can show up in multiple places in a braintree response.
Now we can get it from the other places.
DIBS Payment Services

This commit is maintained separate until DIBS setup a sandbox.
Code by @maxwhite