线程：
1.异步线程全局队列
dispatch_async(dispatch_get_global_queue(0, 0), ^{
});
2.主线程
dispatch_async(dispatch_get_main_queue(), ^{
});
队列
1.串行队列
dispatch_queue_t queue = dispatch_queue_create("tk.bourne.testQueue", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
dispatch_async(queue, ^{
});
