BridgePay: Add support for verify

I don't see any zero dollar auth capability in BridgePay; underneath the
covers, we do a $1.00 auth followed by a void.

If the void fails, the overall transaction still succeeds (like the
verify operation for other gateways).  In BridgePay's case, this is
particularly useful since we've been told that some of BridgePay's
backend processors (like First Data Omaha) do not allow voids of
authorizations.