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
 
If quickbooks is the only integration using this gem right now, this should be optional for people who are using ActiveMerchant but not Quickbooks. [Here's how the braintree integration does it](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/billing/gateways/braintree_blue.rb#L3-L7).