title: change bootstrap div class with jquery button click
body: <p>when page loading i want to show 2 col-md-6 div and one button saying "click to show third div". if someone click the button, i want to toggle third div col-md-4 and change previous 2 col-md-6 to col-md-4. Change button to "click to hide 3 rd div"</p>
  
  <p><strong>HTML</strong></p>
  
  <pre><code>&lt;div class="container"&gt;
  &lt;h1&gt;Hello World!&lt;/h1&gt;
  &lt;div class="row pull-right" style="margin-bottom:10px;"&gt; &lt;button class="btn btn-primary" id="btncontrol"&gt;Click to Add &lt;/button&gt; &lt;/div&gt;
  &lt;div style="clear:both"&gt;
      &lt;div class="row"&gt;
          &lt;div class="col-sm-6" id="col_fld_6" style="background-color:lavender;"&gt;.col-sm-4&lt;/div&gt;
          &lt;div class="col-sm-6" id="col_fld_62" style="background-color:lavenderblush;"&gt;.col-sm-4&lt;/div&gt;
          &lt;div class="col-sm-4" style="background-color:#E6B2A4;display:none" id="last_div"&gt;.col-sm-4&lt;/div&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>