title: Getting NSDictionary keys sorted by their respective values
body: <p>I have an <code>NSMutableDictionary</code> with integer values, and I'd like to get an array of the keys, sorted ascending by their respective values. For example, with this dictionary:</p>
  
  <pre><code>mutableDict = {
      "A" = 2,
      "B" = 4,
      "C" = 3,
      "D" = 1,
  }
  </code></pre>
  
  <p>I'd like to end up with the array <code>["D", "A", "C", "B"]</code>. My real dictionary is much larger than just four items, of course.</p>