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
 
It doesn't. This just calculates the Signature Base String for us, whereby the parameters of the request need to be known to form the signature, but at this point any external calls have already been made (by virtue of having the access token credentials at this point already.)