More Integration Tests for CreditCard/Customer

TODO: Figure out how to deal with the sandbox having the same
customer ids once a live run has been done.  The problem right now
is that when you delete your local recordings, a lot of the live
integration tests will fail again as the "customer id" has been taken.

If you purge your sandbox and set it back up, all integration tests
will pass again.  That kind of sucks...  Have to give this more thought
yet.

 modified:   credit_card_gateway.go
 modified:   credit_card_test.go
 modified:   customer_test.go
 modified:   test_helpers.go