Add OrderReferenceObject model to help with Off Amazon Payment API

# What
Adds a new class to model Amazon's ORO. Primarily, this model's responsibility is to fetch an ORO by ID and expose a few convenience methods for retrieving response data.

# Testing
1. Login via any "Login with Amazon" widgets to the point where the address book appears.
2. That just created an ORO, retrieve its ID by looking at `orderReference.getAmazonOrderReferenceId()` in `onOrderReferenceCreate` in Amazon's JS.
3. Instantiate and play with the ORO. `oro = Peddler::OrderReferenceObject.new('<that ORO ID>')`. Pass in an `MWS::OffAmazonPayments` instance with the MWS tokens all configured unless you have them set in environment variables.

# Todo
- [ ] Deal with MultiXML/Nokogiri conflict

WIP, don't merge