Upgrade FedEx ship API to v12; fix CustomerReferences XML element

This fixes some issues I discovered today from my previous pull request #31.

First of all, the Fedex Web Services ship API was using version 10, which includes some but not all of the CustomerReferenceType enum values that were documented in #31. Upgrading to v12 fixes this, but it required some changes to the paying-party element in the ship request.

Further, there was an error in the way I handled multiple customer reference numbers based on my misreading of the WSDL. Each customer reference number must have its own `<CustomerReferences>` element. The fixed code has been tested against the Fedex development servers (gatewaybeta.fedex.com) and appears to be working for me.

Thanks!
Brad