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.
`utils.invokeCallback` invokes callback for current callout only.
`utils.invokeEventCallback` invokes callbacks for current callout and current tour.

Since it's a single call out (not part of a tour), we want to call only one callback. I believe we'd want to use `utils.invokeCallback` here instead of `utils.invokeEventCallback`.

Using `utils.invokeEventCallback` might cause issues when single callout is shown\closed independently  in the middle of a running tour.

Could you try using this instead:

```
  if (this.opt.onShow) {
    utils.invokeCallback(this.opt.onShow);
 }
```