title: an NSPredicate for selecting a row with maximum value by group
body: <p>I'm using Core Data to store entities in the form</p>
  
  <p>TrackerEntry</p>
  
  <ul>
  <li>username</li>
  <li>timestamp</li>
  </ul>
  
  <p>I want to select the latest record for each user. Using sql it would be something like</p>
  
  <pre><code>SELECT MAX(timestamp) FROM Log GROUP BY username
  </code></pre>
  
  <p>Is there anyway to create an NSPredicate to do this?</p>