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 
- [x] implement some pattern for generating a GUID
 
Can you name this `add_charge_data`? Even though in aggregate it's making calls to the `add_*` methods, seeing it inside a `purchase` method without the `add_` prefix made me wonder what it was doing.