苹果自带的刷新控件UIRefreshControl(不推荐使用)
有bug，例：如果正在刷新，用tabbar切换控制器界面时会卡住
UIRefreshControl *control = [[UIRefreshControl alloc] init];
[control addTarget:self action:@selector(loadNewTopics) forControlEvents:UIControlEventValueChanged];
[control beginRefreshing];
[self.tableView addSubview:control];
