cocos2dx SpriteBatchNode 精灵的渲染优化类

用处是减少对精灵的渲染次数，用法如下

    SpriteFrameCache::getInstance()->addSpriteFramesWithFile("person.plist");
    SpriteBatchNode *node = SpriteBatchNode::create("person.png");
    bg->addChild(node);
    
    SpriteFrame *frame = SpriteFrameCache::getInstance()->getSpriteFrameByName("navigationbar_back.png");
    Sprite *spt1 = Sprite::createWithSpriteFrame(frame);
    
    node->addChild(spt1);