title: Feature ID's in OpenLayers 3 are undefined
body: <p>In OL2 feature id's were generated internally. In OL3 this doesn't seem to be the case since the id's are undefined. I have tried with WKT, JSON, and KML sources and none of them have id's assigned to features. Has the internal feature id been removed from OL3? If not what do I need to do to get the id's generated. I am requesting the id via:</p>
  
  <pre><code>feature.getId();
  </code></pre>
  
  <p>This is real problem for drag and drop since I don't have control over the files.</p>
  
  <p>Cheers</p>