Added tax_category dropdown to New Product Form

(Resubmitted in cleaner branch)

This relates to issue #269

-Added tax_category dropdown to New Product form to make sure it's not overlooked when adding new products
-Set up validation for the tax_category dropdown
-Checked to see if it saved the data properly
-Added an if statement with the dropdown so that if there's only one tax category for that tax zone it selects it automatically to save the user some time, otherwise it inserts blank option as default and requires selection of a specific tax category.
-Included Rob's code and Raf's suggestions
-Wrote spec files, tested, rewrote spec files