Added tax_category dropdown to New Product Form

First commit! This relates to the bits discussed in 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.
Hi Matt. We shouldn't need to declare tax_category_id here as it is already declared on the existing Spree::Product model. Forgive me if this is telling you something you already know, but models in our repository that end in "_decorator" are adding functionality to existing models in the Spree repository, so it is useful to check there first to understand the base you are building from.