Added unit test to check that tooltips is not showed when leave event is triggered before show delay has expired AND the hide delay is set to 0

Cancel running timer for tooltips with delayed show, but instant hide. This prevents delayed tooltips from appearing if the mouse leaves the elements before tooltip is showed and the hiding delay is 0.

all add a few notes to js readme about updated event

If you had a custom class to the root object twipsy would overwrite
that class ($tip[0].className = 'twipsy'), now this has been changed
so it removes only classes that twipsy adds.