title: bootstrap 3 push/pull issue
body: <p>I am learning to use bootstrap 3</p>
  
  <p>I have a problem with using push and pull
  Normally the rows are centered
  but When i use push/pull the row gets aligned left, and go off the left of the screen if the browser is not fullscreen.</p>
  
  <p>why is this</p>
  
  <p>code bellow</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
  &lt;meta http-equiv="content-type" content="text/html; charset=windows-1252"&gt;
      &lt;title&gt;Bootstrap 3 Playabout&lt;/title&gt;
      &lt;meta name="viewport" content="width=device-width, initial-scale=1.0"&gt;
      &lt;META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW"&gt;
      &lt;link rel="stylesheet" href="styles.css"&gt;
      &lt;script src="bootstrap.js"&gt;&lt;/script&gt;
  
      &lt;style&gt;
      .a
      {
          background-color:green;
      }
  
      .b
      {
          background-color:blue;
      }
      &lt;/style&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
      &lt;div class="container"&gt;
          &lt;div class="row"&gt;
              &lt;div class="col-xs-6 col-md-4 a col-lg-push-4"&gt;One&lt;/div&gt;
              &lt;div class="col-xs-6 col-md-8 b col-lg-pull-8"&gt;Two&lt;/div&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/body&gt;
  </code></pre>