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`)
Please always use `example.com` for instance `cancel.example.com`.