title: iOS: multiline uilabel only shows one line with autolayout
body: <p>The code below, I think should result iin a multiline label followed by a button, however, after layout there is only a single line of the label showing up. While I could put an explicit height in the vertical layout that would defeat the purpose. Any ideas about what other constraints I should apply?</p>
  
  <pre><code>UILabel *lbExplain = [[UILabel alloc] init];
  lbExplain.text = @"The Sync process allows you to synchronize your library between different devices. By clicking on the button below you can find other devices to sync with. The other device also has to be running this applicaton.";
  lbExplain.lineBreakMode = NSLineBreakByWordWrapping;
  lbExplain.numberOfLines = 0;
  lbExplain.translatesAutoresizingMaskIntoConstraints = NO;
  
  UIButton *btnPartner = [UIButton buttonWithType:UIButtonTypeRoundedRect];
  [btnPartner setTitle:@"Look for Partners" forState:UIControlStateNormal];
  [btnPartner addTarget:self action:@selector(findPartners:) forControlEvents:UIControlEventTouchUpInside];
  btnPartner.translatesAutoresizingMaskIntoConstraints = NO;
  
  [self.view addSubview:lbExplain];
  [self.view addSubview:btnPartner];
  
  [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[lbExplain]-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(lbExplain)]];
  [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[lbExplain]-[btnPartner]" options:NSLayoutFormatAlignAllLeft metrics:nil views:NSDictionaryOfVariableBindings(lbExplain, btnPartner)]];
  </code></pre>