pay gate refund function

Added refund function
Added test case for refund function

Please communicate any issues to stephan@g2design.co.za

This is the first code i have written in ruby and i am a little bit shocked that my test case passed at first go.
Please confirm if i in fact did right the test case correctly

I think you want this test to be in the remote tests for the gateway, not the unit tests - your `test_successful_refund` is already doing the unit testing portion of the refund integration.