蘋果自帶的刷新控件UIRefreshControl(不推薦使用)
有bug，例：如果正在刷新，用tabbar切換控制器界面時會卡住
UIRefreshControl *control = [[UIRefreshControl alloc] init];
[control addTarget:self action:@selector(loadNewTopics) forControlEvents:UIControlEventValueChanged];
[control beginRefreshing];
[self.tableView addSubview:control];
