Protect from firing an event several times after show/hide of matched element

Hi, Daniel!
I started to use your awesome hack and faced with one problem: if you hide an matched element and then show it again, the element's `addSelectorListener` handler retrieves an event. The is because of css animations nature. In my opinion this behaviour is a little bit confusing, because usually we expect the event only once, when an appropriate element is  inserted into DOM.

As a result see my fix for the problem.

Thanks,
Maksim