Callbacks on callouts do not work

Bug Issue #97 The (registerHelper or listen) do not work as intended for onClose/'close' and onShow/'show' for callouts.
`hopscotch.listen` should trigger for standalone callouts as well as tours.  We need to have that fixed as well. However, using `utils.invokeEventCallback` will introduce some issues, as I mentioned earlier. We are planning to do a larger refactor\clean up of global\tour\callout level configs and will address this issue then. Meanwhile, does adding `utils.invokeCallback(this.opt.onShow);` sounds like a reasonable compromise?