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.
A very subtle difference to notice here. If you call invokeEventCallbacks (without opt.onShow) then the callout level registerHelper do not get fired. Only the tour level 'show' gets fired. Hence it is necessary to pass opt.onShow and if that is null or undefined, then the listen('show', callback) is called.