title: How to present a splash/login view controller using storyboards
body: <p>I've seen this question in various forms without a clear answer.  I'm going to ask and answer here.  My app needs to do work on startup... inits, a few network calls, login, etc.  I don't want my main view controller working until that's done.  What's a good pattern for doing this?</p>
  
  <p>Requirements:</p>
  
  <ul>
  <li>iOS5+, storyboards, ARC.</li>
  <li>the main vc's view cannot appear until the startup work is done.</li>
  <li>want a choice of transition styles to the main vc when the startup work is done.</li>
  <li>want to do as much layout as possible in storyboard.</li>
  <li>the code should be cleanly contained somewhere obvious.</li>
  </ul>