Centered top and bottom tooltips above or below an element.

Centered top and bottom tooltips by adding the classes tip-centered-top and tip-centered-bottom along side has-tip. The tooltip is positioned centered whether the tooltip is wider or narrower than the target element.  The nub is positioned in the middle of the tool tip. 

    <span class="has-tip tip-centered-top" data-width="120" title="I'm a centered tip above the target element.">tip-centered-top</span>

    <span class="has-tip tip-centered-bottom" data-width="120" title="I'm a centered tip below the target element.">tip-centered-top</span>

![centered-tooltip-bottom](https://f.cloud.github.com/assets/647691/44738/d9ba83ca-56f1-11e2-87ea-d9a3a2d95e9c.png)