Force hover leave on new mouseenter. Fixes #63 and #64.

If the mouse has entered a new element, this change forces the hoverleave event to fire on the previously hovered element, even if no mouseleave event fires.

The mouseleave event will not fire if the element is removed while hovered and possibly in other circumstances.
Merge pull request #65 from Spredfast/fix-hoverleave

Force hover leave on new mouseenter. Fixes #63 and #64.