added layer throttling which greatly improves drag and drop and other mousemove drawing performance

src/Layer.js

@@ -10,8 +10,15 @@
  * @param {Object} config
  */
 Kinetic.Layer = function(config) {
+    // default attrs
+    if(this.attrs === undefined) {
+        this.attrs = {};
+    }
+    this.attrs.throttle = 12;
+
     this.nodeType = 'Layer';
-    
+    this.lastDrawTime = 0;
+
     this.canvas = document.createElement('canvas');
     this.context = this.canvas.getContext('2d');
     this.canvas.style.position = 'absolute';
@@ -29,7 +36,28 @@ Kinetic.Layer.prototype = {
      *  or shapes
      */
     draw: function() {
-        this._draw();
+        var throttle = this.attrs.throttle;
+        var date = new Date();
+        var time = date.getTime();
+        var timeDiff = time - this.lastDrawTime;
+
+        if(timeDiff >= throttle) {
+            this._draw();
+            this.lastDrawTime = time;
+        }
+    },
+    /**
+     * set throttle
+     * @param {Number} throttle in ms
+     */
+    setThrottle: function(throttle) {
+    	this.attrs.throttle = throttle;
+    },
+    /**
+     * get throttle
+     */
+    getThrottle: function() {
+    	return this.attrs.throttle;
     },
     /**
      * clears the canvas context tied to the layer.  Clearing