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.
I'd like to find a way to make this optional, based on the particular deployment. For example, in Australia, fresh food is tax exempt, and so forcing us to set a tax category for every product doesn't really make sense.

I think we should use a global config variable (require_tax_category...?) that could be set up using [Spree's existing Preferences functionality](http://guides.spreecommerce.com/developer/preferences.html). I will have a bit more of a think about how to implement this and get back to you ASAP.