Add most of the unit tests for fabric.util methods. Fix fabricjs#35.

all.js

@@ -5122,6 +5122,8 @@ fabric.util.string = {
      * Indicates whether fabric.Canvas#add should also re-render canvas.
      * Disabling this option could give a great performance boost when adding a lot of objects to canvas at once
      * (followed by a manual rendering after addition)
+     * @property
+     * @type Boolean
      */
     renderOnAddition: true,
 
@@ -5385,12 +5387,20 @@ fabric.util.string = {
       return this.lowerCanvasEl;
     },
 
-    // placeholder
+    /**
+     * Returns currently selected object, if any
+     * @method getActiveObject
+     * @return {fabric.Object}
+     */
     getActiveObject: function() {
       return null;
     },
 
-    // placeholder
+    /**
+     * Returns currently selected group of object, if any
+     * @method getActiveGroup
+     * @return {fabric.Group}
+     */
     getActiveGroup: function() {
       return null;
     },
@@ -5420,6 +5430,7 @@ fabric.util.string = {
      * Adds objects to canvas, then renders canvas;
      * Objects should be instances of (or inherit from) fabric.Object
      * @method add
+     * @param [...] Zero or more fabric instances
      * @return {fabric.Canvas} thisArg
      * @chainable
      */