fix uuid generator, the first 0 was missing

src/jio/jio.intro.js

@@ -2,9 +2,13 @@ var jio = function(spec) {
 
     var generateUuid = function () {
         var S4 = function () {
-            return Math.floor(
+            var i, string = Math.floor(
                 Math.random() * 0x10000 /* 65536 */
             ).toString(16);
+            for (i = string.length; i < 4; i += 1) {
+                string = '0'+string;
+            }
+            return string;
         };
         return S4() + S4() + "-" +
             S4() + "-" +