title: UIScrollView and constraints issue
body: <p>I create a UIScrollView:</p>
  
  <pre><code>// datasetSubView.m
  UIScrollView *scrollView = [UIScrollView new];
  scrollView.translatesAutoresizingMaskIntoConstraints = FALSE;
  scrollView.userInteractionEnabled = TRUE;
  scrollView.scrollEnabled = TRUE;
  scrollView.backgroundColor = [UIColor whiteColor];
  scrollView.alpha = 0;
  
  self.filterListView = scrollView;
  </code></pre>
  
  <p>Then in my UIViewController, I add the scroll view and give it a size:</p>
  
  <pre><code>[self.view addSubview:self.datasetSubBar.filterListView];
  
  [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[filterListView]|" options:0 metrics:nil views:@{@"filterListView": self.datasetSubBar.filterListView}]];
  [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[filterListView][filtersSubBar]" options:0 metrics:nil views:@{@"filterListView": self.datasetSubBar.filterListView, @"filtersSubBar" : self.datasetSubBar.filtersSubBar}]];
  
  [self.view layoutIfNeeded]; //gives the filterListView its frame
  </code></pre>
  
  <p>I then create a custom UIView to place in the scroll view:</p>
  
  <pre><code>[self.datasetSubBar createPanels];
  
  [self.datasetSubBar.filterListView layoutIfNeeded]; // w/out this, the panel's size is 0
  
  //datasetSubBar.m
  -(void)createPanels {
      DatasetFilterListPanelView *panel = [[DatasetFilterListPanelView alloc] init];
      panel.translatesAutoresizingMaskIntoConstraints = FALSE;
      panel.headerLabel.text = [NSString stringWithFormat:@"Panel %d", 1];
      panel.tag = 1;
  
      [self.filterListView addSubview:panel];
  
      [self.filterListView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[panel]" options:0 metrics:nil views:NSDictionaryOfVariableBindings(panel)]];
      [self.filterListView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:[panel]-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(panel)]];
  
      self.panels = @[panel];
  }
  </code></pre>
  
  <p>It should place the subView in the top right corner, but it doesn't:</p>
  
  <pre><code>NSLog(@"viewDidAppear | self.datasetSubBar.filterListView: %@", self.datasetSubBar.filterListView);
  
  NSLog(@"self.datasetSubBar.panels: %@", self.datasetSubBar.panels)
  </code></pre>
  
  <p>.</p>
  
  <pre><code>2013-02-22 16:28:22.912 [5196:14003] viewDidAppear | self.datasetSubBar.filterListView: &lt;UIScrollView: 0x8489f90; frame = (0 0; 768 934); clipsToBounds = YES; alpha = 0.8; gestureRecognizers = &lt;NSArray: 0x848a650&gt;; animations = { opacity=&lt;CABasicAnimation: 0x98138b0&gt;; }; layer = &lt;CALayer: 0x848a160&gt;; contentOffset: {0, 0}&gt;
  2013-02-22 16:28:22.913 [5196:14003] self.datasetSubBar.panels: (
      "&lt;DatasetFilterListPanelView: 0x9853570; frame = (-385 20; 365 165); tag = 1; layer = &lt;CALayer: 0x9853640&gt;&gt;"
  </code></pre>
  
  <p>It feels like at the time of the subView's creation, the filterListView's width is 0 cause if the superView was 768, the subView anchor should be (383, 20) {superView's width (768) - subView's width(365) - padding(20)}.</p>
  
  <p>This is frustrating me, it should work. Do UIScrollViews act differently with constraints?</p>
  
  <p>EDIT:</p>
  
  <p>Yes, UIScrolView handles constraints differently; I changed the scroll view to a UIView and works like it should.</p>