allow followMouse tooltips to be above the mouse pointer

This library looks great, but I can't get over the followMouse tooltips being below the mouse pointer, it just looks wrong to me... so here's a patch that allows you to have them above the mouse by setting `options.offsetUp: true` (defaults to `false`). This positions the tooltip so that the bottom edge is `options.offset` (positive integer) pixels above the mouse pointer.

I've also horizontally centered the tooltips with the mouse pointer in the center and changed the collision behaviour a little.

These changes (should) only affect tooltips where `options.followMouse: true` and should 'just work', try setting `options.offsetUp: true` on a page that already has followMouse tooltips to see the difference.

I hope you like these changes, if you don't just bin them.