Allow PaymentsGateway payments of $0.00

This allows us to checkout orders of $0.00 with StyleSeat Checkout, by skipping the SOAP calls to authorize and capture the payment if the variant's amount is zero.

Assigned @greggarson   
Do we want to check that the amount charged was 0 here too and then let things blow up as they would have previously if there is actually no authorization_code and pg_trace_number