Add isTracking accessor method

Backbone.Undo.js

@@ -638,6 +638,13 @@
 			this.set("track", false);
 			this.stack.track = false;
 		},
+        /**
+         * Return the state of the tracking
+         * @return {boolean}
+         */
+        isTracking: function () {
+            return this.get("track");
+        },
 		/**
 		 * This is the "all"-handler which is bound to registered 
 		 * objects. It creates an UndoAction from the event and adds