title: NSDateFormatter to parse ISO8601 with and without milliseconds
body: <p>Is it possible to have a <code>NSDateFormatter</code> parse both an ISO8601 date with and without milliseconds.</p>
  
  <p>I have the following 2 date formats being returned by an API and sometimes they have milliseconds included and sometimes not.</p>
  
  <p>For example I might have <code>2013-07-23T13:45:02Z</code> and <code>2013-07-23T13:45:02.677Z</code>. For the former setting the <code>dateFormat</code> property to <code>@"yyyyMMdd'T'HHmmss'Z'"</code> works fine.</p>
  
  <p>Is there a format I can use to parse the later, optionally parsing the milliseconds if they exist?</p>