Add optional ReferenceNumber to package options.

ReferenceNumbers can be included with a shipping package. They contain a
code which needs to have one of the accepted values as per Appendix I of
the Shipping Package XML Developer Guide and a value. In addition the
value of the ReferenceNumber can be generated as a barcode.

Change-Id: I28cfa8db046847f600517c0ea8d3673944a1a39f
Merge pull request #2 from freerunningtech/ups-references

Add optional ReferenceNumber to package options.