WIP: Allow rendering public field with Field API formatter

(Completely untested, and probably doesn't work yet)
Move [this](https://github.com/Gizra/restful/blob/field-formatter/plugins/restful/RestfulEntityBase.php#L330-L350) to a helper method.

```php
protected function valueFromFieldFormatter($value, $property, $display, $multiple) {
  // Get values from the formatter.
  $output = field_view_field($this->getEntityType(), $value, $property, $display);
  if ($info['formatter']['remove_theme']) {
    // Unset the theme, as we just want to get the value from the
    // formatter, without the wrapping HTML.
    unset($output['#theme']);
  }
  if ($multiple) {
    // Multiple values.
    $deltas = array_keys($output);
    foreach (element_children($deltas) as $delta) {
      $value[] = drupal_render($output[$delta]);
    }
  }
  else {
    // Single value.
    $value = drupal_render($output);
  }
}
```