Changes to Barcode.php

I've made several changes to Barcode.php:
-ean8 barcodes were not generating the correct check digit due to a - instead of a + in the logic, fixed, and they now verify using online ean8 verification tools.
-added code39 barcodes, A-Z, 0-9, some special characters.
-Changed logic for generating ean barcodes to improve speed. In my test it resorted in faster barcode creation for 100000 ean barcodes.
-Added a ref to the readme for the new code39 generation
When I tested it, it was faster this way, but that may not be true in your tests.