QuickBooks Payments 

WIP to udpate quickbooks payments .

Basic integration is complete, items remaining:
- [ ] update integration to authenticate via OAuth
- [ ] authentication testing 
- [ ] remote tests for Quickbooks 
- [ ] some unit tests 

Worth looking into how to generalize OAuth for use by all gateways.
 
Do these credentials actually work? It seems like `initialize` only requires `request_id`, which I don't see here..