Fix mouseout on nested elements

`on('mouseout')` triggers for nested children of the target
element. This causes tooltips to flicker if they're attached
to an element with nested spans eg. `no_translate`.

This fix uses the jquery event listener `mouseleave` to
stop propagation of the children mouse out events.
Merge pull request #5 from 99designs/fix-mouseout-with-nested

Fix mouseout on nested elements