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.
Sounds good. I heard you might have different tax categories on different items in Aus as well, so I thought it might be a similar deal. 

I made it so the main tax category is automatically selected if there is only one set for that region, as an attempt to make it easier for regions with simpler tax frameworks. Having a global  variable seems like a better solution though. I'll try to check it out tonight, I'm working at our food hub today. Thanks for the pointers.