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
Add ability to override address validation for UPS

Previously address validation would be enabled always. This provides a
way to pass an option parameter to override that.

Change-Id: I4d63802619eb80d07e750b5212ce9dd5bb0bc8c9
Check for config file existance before loading.

Previously if omniship.yml did not exist in config folder of application
an Errno::ENOENT error would be raised. This change checks for file
existance before attempting to load_file. This is needed so settings can be
passed as options to carrier, without the need for a config file.

Change-Id: I0c77708250b147b3825ea53b1a9dd1bd34b9e4f0
Merge pull request #4 from freerunningtech/optional-config

Check for config file existance before loading.
Merge pull request #3 from freerunningtech/ups-address-validation

Add ability to override address validation for UPS
Merge pull request #2 from freerunningtech/ups-references

Add optional ReferenceNumber to package options.