Added UMD definition to knockout-sortable

Fixes #488

knockout-sortable.js

@@ -1,4 +1,17 @@
-(function () {
+(function (factory) {
+    "use strict";
+    if (typeof define === "function" && define.amd) {
+        // AMD anonymous module
+        define(["knockout"], factory);
+    } else if (typeof require === "function" && typeof exports === "object" && typeof module === "object") {
+        // CommonJS module
+        var ko = require("knockout");
+        factory(ko);
+    } else {
+        // No module loader (plain <script> tag) - put directly in global namespace
+        factory(window.ko);
+    }
+})(function (ko) {
     "use strict";
 
     var init = function (element, valueAccessor, allBindings, viewModel, bindingContext, sortableOptions) {
@@ -158,4 +171,4 @@
         }
     };
 
-})();
+});