title: How to center a width unknown button in a bootstrap's span?
body: <p>I have a button generated by backbone.js and jQuery.tmpl() and width of the button is variant. I want to put it in a span6 div. Code is shown below:</p>
  
  <pre><code>&lt;div class="row"&gt;
    &lt;div class="span6"&gt;
      &lt;button class="btn primary large"&gt;
        BLABLABLA
      &lt;/button&gt;
    &lt;/div&gt;
    &lt;div class="span10"&gt;
      ...
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I don't know how to make the button centered in the div.</p>