QuickBooks Payments 

WIP to integrate QuickBooks Payments.

Basic integration is complete, items remaining:
- [x] update integration to calculate OAuth signature given all token credentials
- [ ] remote tests for Quickbooks 
- [ ] unit tests 
- [ ] implement some pattern for generating a GUID
 
*sigh* OAuth seems like such a bad choice for this type of API. But c'est la vi. Guess we'll note it as a dep for the Quickbooks gateway, then.