Re-add tip-center to the tooltips.

I did a little bit of work to the javascript, and CSS to re-enable centered tooltips.  It was added [once before](https://github.com/zurb/foundation/commit/3d0832d81703011cbfd8505d56fb3d7650eb9678) and I'm not sure why it's not part of the tooltip library anymore.

I decided that tip-centered could be supplementary to the existing 2 vertical classes instead of require 2 new else ifs.

I've rigged up a [test page on jsFiddle](http://jsfiddle.net/3hJaN/1/) that shows the addition.