[FIX] barcodes: comply the ean parser with reportlab.graphics.barcode

Before this commit, the JS parser would add 0 after the input, then add the ean key.
Whereas reportlab.graphics.barcode fills the input with 0's on the left, then add the key

http://pydoc.net/Python/reportlab/2.5/reportlab.graphics.barcode.eanbc/
At: Ean13BarcodeWidget.__init__()

This commit makes the ean parser coherent with the lib the python uses

addons/barcodes/static/src/js/barcode_parser.js

@@ -102,7 +102,7 @@ var BarcodeParser = Class.extend({
         ean = ean.substr(0,13);
 
         for(var n = 0, count = (13 - ean.length); n < count; n++){
-            ean = ean + '0';
+            ean = '0' + ean;
         }
         return ean.substr(0,12) + this.ean_checksum(ean);
     },