title: set value yiibooster datePickerGroup
body: <p>my code:</p>
  
  <pre><code>&lt;?php echo $form-&gt;datePickerGroup(
              $model,
              'to_date',
              array(
                  'widgetOptions' =&gt; array(
                      'options' =&gt; array(
                          'language' =&gt; 'en',
                          'format' =&gt; 'yyyy-mm-dd',
                      ),
                  ),
                  'wrapperHtmlOptions' =&gt; array(
                      'class' =&gt; 'col-sm-5',
                  ),
                  'hint' =&gt; 'Click inside! This is a super cool date field.',
                  'prepend' =&gt; '&lt;i class="glyphicon glyphicon-calendar"&gt;&lt;/i&gt;'
              )
          ); ?&gt;
  </code></pre>
  
  <p>my to_date is timestamp
  i want befor view value convert value by: <code>date("Y-m-d",$model-&gt;to_date)</code>  </p>