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 add this to Gemfile_common too? https://github.com/Shopify/active_merchant/blob/master/Gemfile_common