Improving error details
Merge pull request #30 from geermc4/master

Improve exception details
Allow customer references to have arbitrary type
Merge pull request #31 from acumenbrands/master

Allow customer references to have arbitrary type
Update Readme.md
Upgrade FedEx ship API to v12
Fix CustomerReferences XML element
Tracking spec: Unique IDs are account-specific; make this generalized
Payor element depends on API version of particular request
Fix track test