HOTFIX 2015.4, 2015.3 - Added a $$ITEMGTIN$$ merge for invoices and packing slips that can be used in the same places as $$VARIATIONINFOS$$

Added a $$ITEMGTIN$$ merge for invoices and packing slips that can be used in the same places as $$VARIATIONINFOS$$. Outputs "GTIN: " followed by either the order item, variant inventory item, or the product's GTIN (in that order) if one is specified. Otherwise outputs nothing. Additionally the same GTIN output by $$ITEMGTIN$$ now shows up on the viewOrder page in the admin.