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
 
Basically, `OAuth::Client::Helper#header` does [all this stuff](http://nouncer.com/oauth/authentication.html) for us.