change mouseover/mouseout to mouseenter/mouseleave

Fix bug when hovering one item with dark-tooltip that has an child node like an icon. The plugin trigger multiples events when hovering the child node making the tooltip hide n show, flashing. Use the mouseenter and mouseleave events fix the bug.

See the exemple of the behavior: http://api.jquery.com/mouseover/
Merge pull request #4 from tonypine/master

Bug: change mouseover/mouseout to mouseenter/mouseleave