Add option for nontaxable items in Online Store

Not all items we sell on the online store are taxable in Connecticut. We need to be able to mark them as nontaxable so parents don't overpay. If someone is charged sales tax, even incorrectly, we have remit it to the state. If we back out the tax and set the item price low enough that adding tax brings it to the sales price we will still have to remit that tax to the state. Refunding the tax to parents would be too time consuming.