Validates GTIN (Global Trade Item Number) codes by calculating checksums.
GTIN comprises of GTIN-8, GTIN-12, GTIN-13 and GTIN-14 codes. EAN, UPC and ISBN can be thought of as subsets of GTIN. For more information, see: http://www.gs1.org/barcodes/technical/idkeys/gtin and http://en.wikipedia.org/wiki/Global_Trade_Item_Number