make the extension for $.Callbacks optional to support jQuery 1.6.*

Author: cburgdorf

lib/rx.jquery.js

@@ -43,21 +43,24 @@
             deferred.reject(e);
         });
         return deferred;  
-    };
-
-    $.Callbacks.prototype.toObservable = function () {
-        var parent = this;
-        return observableCreate(function (observer) {
-            var handler = function(values) {
-                observer.onNext(values);
-            };
-            parent.add(handler);
-            return function () {
-                parent.remove(handler);
-            };
-        });
     };
 
+    //in order to support jQuery 1.6.*
+    if ($.Callbacks) {
+        $.Callbacks.prototype.toObservable = function () {
+                var parent = this;
+                return observableCreate(function (observer) {
+                    var handler = function(values) {
+                        observer.onNext(values);
+                    };
+                    parent.add(handler);
+                    return function () {
+                        parent.remove(handler);
+                    };
+                });
+            };
+    }
+
     proto.onAsObservable = function (events, selector, data) {
         var parent = this;
         return observableCreate(function(observer) {