Expose #className on all classes that define #_class

Closes #576

src/core/Base.js

@@ -36,6 +36,16 @@ Base.inject(/** @lends Base# */{
             }, []).join(', ') + ' }';
     },
 
+    /**
+     * The class name of the object as a string, if the prototype defines a
+     * `_class` value.
+     *
+     * @beans
+     */
+    getClassName: function() {
+        return this._class || '';
+    },
+
     /**
      * Serializes this object to a JSON string.
      *

src/item/Item.js

@@ -320,13 +320,10 @@ var Item = Base.extend(Emitter, /** @lends Item# */{
     /**
      * The class name of the item as a string.
      *
+     * @name Item#className
      * @type String('Group', 'Layer', 'Path', 'CompoundPath', 'Shape',
      * 'Raster', 'PlacedSymbol', 'PointText')
-     * @bean
      */
-    getClassName: function() {
-        return this._class;
-    },
 
     /**
      * The name of the item. If the item has a name, it can be accessed by name