don't register an unload listener

[erikvold]

Firefox has a bfcache which is not used when the page registers an unload listener. If there is a way to destroy listeners then that would probably be enough, and https://github.com/marquete/kibo/blob/master/kibo.js#L140 could be removed.

[marquete]

Hello, Erik. (: Thanks for your feedback - I didn't know that about Firefox. I'll get to this and other issues as soon as I have some free time. Cheers!