title: Detect if a transclude content has been given for a angularjs directive
body: <p>I have a directive (a progressbar) which should have two possible states, one without any description and one with a label on the left side. 
  It would be cool to simple use the transcluded content for this label.</p>
  
  <p>Does anyone know how I can add a class to my directive depending whether a transclude content has been given or not?</p>
  
  <p>So I want to add:</p>
  
  <pre><code>&lt;div class="progress" ng-class="{withLabel: *CODE GOES HERE*}"&gt;
      &lt;div class="label"&gt;&lt;span ng-transclude&gt;&lt;/span&gt;
      &lt;div class="other"&gt;...&lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Thanks a lot!</p>