Dummy 3D Secure provider

Added `Dummy3DSecureProvider` that allows to simulate 3D Secure verification redirects during payment
With this provider following integration tests can be created:
* Gateway redirects to 3DSecure verification page (various verification results can be used)
* Gateway is unavailable (raises `URLError`)
* Gateway returns unsupported response (raises `PaymentError`)
I still don't understand why we have two dummy Providers here. Now `DummyProvider` is one case of `Dummy3DSecureProvider`.

For backwards functionality I propose to use defaults in new form that will be acting like old one.