title: UIStackView - Want a percentage to define each item
body: <p>I want to put two views into a UIStackView. I want the top item to always be 30% of the space no matter what device (or orientation) that it appears. Thus, I want to have the bottom view to be 70%. How do I tell a UIStackView that I want to use a percentage?</p>
  
  <p><a href="http://i.stack.imgur.com/ln1v1.png" rel="noreferrer">Example resulting view</a></p>