let nav_bar method support full screen fluid view

The little change let the nav_bar method supports full screen fluid view. After this update, you can set the `:fluid` option to `true`, this will set the class of container div to be "container-fluid'.

````
<%= nav_bar :fluid => true  %>
````

will render

    <div class="navbar">
      <div class="container-fluid">
      </div>
    </div>