Fixed SpriteSheetLoader to work with JSONP (thanks @jonlucas)

Author: Lanny McNie

src/preloadjs/loaders/SpriteSheetLoader.js

@@ -99,7 +99,7 @@ this.createjs = this.createjs || {};
 	// protected methods
 	p._createRequest = function() {
 		var callback = this._item.callback;
-		if (callback != null && callback instanceof Function) {
+		if (callback != null) {
 			this._request = new createjs.JSONPLoader(this._item);
 		} else {
 			this._request = new createjs.JSONLoader(this._item);