Add shop_order, shop_location_note to Category model.
Order shopping list by item.category.shop_order.
Display shop_location_note if available.
Merge pull request #7 from gregwym/shopping-order

Add field to assign Shopping order and location note to each category.
Update item sold_number, used coupon when Invoice has been paid.
Only send order received when Invoice status is not cancelled.
Remove "confirmation" term from the invoice page.
Handle ObjectDoesNotExist exception in invoice view and paypal callbacks.
Merge pull request #8 from gregwym/payment-handling

Fix various issue appears in payment handling strategy.