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'm pretty sure this will make a remote call? Which is definitely not going to fly. Lets instead just have the `oauth_header` value passed in at gateway initialization, or other similar pre-setup of the OAuth relationship.