Fixed the position issue of top tooltips

I've added a small change that fixes the position of top tooltips. Without this fix the triangle is positioned directly above the parent, see the image below.

![tooltipexample](https://f.cloud.github.com/assets/6249209/1909723/ae3b1ed6-7d05-11e3-87a1-19f6b1bfdca2.jpg)

When hovering the green block the triangle gets on top of it. You'll lose focus on the green block and the tooltip will disappear, which results in a 'flickering' effect.
With my fix the tooltip works as expected.

You can also see this behaviour on http://foundation.zurb.com/docs/components/tooltips.html. When you hover the 5 heighest pixels of the first 't' in 'tip-top', the tooltip will flicker.