GCD，分组执行

效果类似于async.parallel函数

dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0,0),^{
    // 子任务1
});
dispatch_group_async(group, dispatch_get_global_queue(0,0),^{
    // 子任务2
});
dispatch_group_notify()(group, dispatch_get_global_queue(0,0){
    // 所有子任务完成以后
});