title: add a button to grid view in yii2
body: <p>i'm a new yii2 developer !
  i made a GridView and the code is shown below :</p>
  
  <pre><code>&lt;?php Pjax::begin(); ?&gt;    &lt;?= GridView::widget([
          'dataProvider' =&gt; $dataProvider,
          'filterModel' =&gt; $searchModel,
          'columns' =&gt; [
              ['class' =&gt; 'yii\grid\ActionColumn'],
              ['class' =&gt; 'yii\grid\CheckboxColumn'],
              ['class' =&gt; 'yii\grid\SerialColumn'],
              'id',
              'countryCode',
              'countryName',
              'currencyCode',
          ],
      ]); ?&gt;
      &lt;?php Pjax::end(); ?&gt;
  </code></pre>
  
  <p>a screenshot of output :
  <a href="http://i.stack.imgur.com/BI6fy.png" rel="nofollow">OUTPUT</a></p>
  
  <p>now i want to have a column contain some button and that button for example open a page or somthing else !
  my problem is how can i create that column ?</p>