title: Javascript, Underscore trying to return the key of an object with the highest value
body: <p>I have an object </p>
  
  <pre><code>obj = { "13-17": { "total_fans": 5 }, 
          "18-24": { "total_fans": 10 }, 
          "25-34": { "total_fans": 15 } 
        }
  </code></pre>
  
  <p>and I want to return from <code>obj</code> the key of the object with the highest <code>total_fans</code>, so in this case it would be <code>25-34</code></p>