Added ability to set gravity on an element-by-element basis

Hey,

I needed the ability to choose the gravity applied to the tooltips on many different buttons bit didnt think it was best to call tipsy for each one. Instead I added a data attribute called 'gravity' to each element which seems to work quite well. Use is like so:

    <a href="#" title="Tooltip text" data-gravity="sw">Anchor Text</a>


All the best,
Tom