title: async task queue doesn't work (SWIFT)
body: <p>I try to execute 2 task in the backgroup in sequence with following code:</p>
  
  <pre><code>var group = dispatch_group_create();
  dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
              println("start task 1");
              NSThread.sleepForTimeInterval(10000);
              });
  dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), {
              println("start task 2");
              NSThread.sleepForTimeInterval(10000);
          });
  
  
  dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
  </code></pre>
  
  <p>When running Task 1 and Task 2 are executed at the same time, not inqequence.</p>
  
  <p>What am I doing wrong ?</p>