Align XML structure of models with sequence defined in XSDs

PR alters the toArray() methods of three models: Product, Inventory, and Pricing. It...

1. Distinguishes required from optional parameters by putting them in separate arrays named $required and $optional.
2. Adjusts the structure of the array returned so that the sequence and arrangement of XML elements matches what is laid out in the XSDs.

PR also updates API version numbers and adds a new element to the Product model, called 'seller-tags'. This is a comma-delimited string of tags that make it easier to search your own product catalog in the Sears portal.