added setDrawFunc() method to Shape so that you can dynamically change the drawing function. added new unit test

src/Shape.js

@@ -133,6 +133,13 @@ Kinetic.Shape.prototype = {
     getStrokeWidth: function() {
         return this.strokeWidth;
     },
+    /**
+     * set draw function
+     * @param {Function} func drawing function
+     */
+    setDrawFunc: function(func) {
+        this.drawFunc = func;
+    },
     /**
      * draw shape
      * @param {Layer} layer Layer that the shape will be drawn on