Allow mapping to the created property

The FeedsNodeProcessor class used this instead of the default mapping:

// ...
  public function setTargetElement(FeedsSource $source, $target_node, $target_element, $value) {
    switch ($target_element) {
      case 'created':
        $target_node->created = feeds_to_unixtime($value, REQUEST_TIME);
        break;
// ...

We should do that, too.