fix(jqLite): correct implementation of mouseenter/mouseleave event

Implement mouseenter/mouseleave event referring to
http://www.quirksmode.org/js/events_mouse.html#link8 and jQuery source
code(not dependent on jQuery).
The old implementation is wrong. When moving mouse from a parent element
into a child element, it would trigger mouseleave event, which should not.
And the old test about mouseenter/mouseleave is wrong too. It just
triggers mouseover and mouseout events, cannot describe the process of mouse
moving from one element to another element, which is important for
mouseenter/mouseleave.

Closes #2131, #1811