QuickBooks Payments 

WIP to integrate QuickBooks Payments .

Basic integration is complete, items remaining:
- [ ] update integration to authenticate via OAuth
- [ ] authentication testing 
- [ ] remote tests for Quickbooks 
- [ ] some unit tests 
- [ ] implement some pattern for generating a GUID
 
I'd actually like to see the dependency on OAuth removed completely; I'd prefer the value for the OAuth header actually be the credential that the gateway is initialized with, if that value can't be generated easily. A gateway should be always making calls using the same OAuth token, so there's no need for AM to get involved in OAuth complexity.