Allow PaymentsGateway payments of 0.00

- Skips auth / confirm calls, so no receipt is created
Check amount for capture, break as usual if no auth/trace number
Compare Decimal to Decimal
Optimize Order.paymentvariant property
Optimize Order.save()
Merge pull request #3 from styleseat/allow-zero-dollars

Allow PaymentsGateway payments of $0.00