added getSize() method to Stage, Rect, and Image. Also updated unit tests

src/Stage.js

@@ -120,6 +120,15 @@ Kinetic.Stage.prototype = {
         this.backstageLayer.getCanvas().width = width;
         this.backstageLayer.getCanvas().height = height;
     },
+    /**
+     * return stage size
+     */
+    getSize: function() {
+        return {
+            width: this.width,
+            height: this.height
+        };
+    },
     /**
      * clear all layers
      */