title: MBProgressHUD is not changing to customView
body: <p>I am running around the issue, that MBProgressHUD should be updating its View with a Checkmark/X for succeeded/failed requests. Somehow this doesnt really work as intended and the update only works after all the code has executed.</p>
  
  <h3>initializing the HUD</h3>
  
  <pre><code>  ...
    MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
    hud.labelText = @"Verifying Credit Card";
    _HUD = hud;
  
    CWAPIClient *client = [CWAPIClient sharedClient];
    client.delegate = self;
    dispatch_queue_t backgroundQueue = dispatch_queue_create("com.clubw.billing", 0);
    dispatch_async(backgroundQueue, ^{
      [client save:billingProfile with:[Address defaultBillingAddress]];
    });
    ....
  </code></pre>
  
  <h3>Callback to process information:</h3>
  
  <pre><code>self POST:[NSString stringWithFormat:@"user/%@/billingprofile", [[Profile defaultProfile] userId]] parameters:params success:^(AFHTTPRequestOperation *operation, id responseObject) {
        dispatch_async(dispatch_get_main_queue(), ^{
          [self.delegate CWAPIClient:self doneVerifyingCreditCard:responseObject];
        });
      } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        NSLog(@"%@", error);
      }];
  </code></pre>
  
  <h3>callback when action is complete</h3>
  
  <pre><code>BOOL success = [[jsonResponse objectForKey:@"success"] boolValue];
    if (success)
    {
  
      NSLog(@"Thread: %@", [NSThread currentThread]);
      dispatch_async(dispatch_get_main_queue(), ^{
  
        UIImage *checkmarkImage = [UIImage imageNamed:@"checkmark.png"];
        UIImageView *checkmarkView = [[UIImageView alloc] initWithImage:checkmarkImage];
  
        _HUD.customView = checkmarkView;
        _HUD.labelText = @"Credit Card Verified!";
        _HUD.mode = MBProgressHUDModeCustomView;
        NSLog(@"Thread: %@", [NSThread currentThread]);
  
        sleep(5);
        [_HUD hide:YES];
      });
  </code></pre>
  
  <p>After the 5 seconds, it simply closed the HUD - and the updated one flashes for a second. 
  It seems, like this is a threading issue - but I cant seem to figure out where it is throwing up.</p>