Floating notices

Add class overlay-header or overlay-maincontent to a notice
````html
<div class="wdn_notice overlay-header duration-10">
    <div class="close">
        <a href="#" title="Close this notice">Close this notice</a>
    </div>
    <div class="message">
        <h4>I'm Floating!</h4>
        <p>This notice has classes <em>overlay-header</em> and <em>duration-10</em> and will vanish on its own.</p>
    </div>
</div>
````

![screen shot 2013-08-14 at 5 27 59 pm](https://f.cloud.github.com/assets/446357/965698/00b020d2-0531-11e3-8a0b-d5317bf9707f.png)