Firefox issue with the closeSlider/openSlider functions

When we added this library to a project, we found that the open/close slider links weren't working in Firefox. Clicking them would just add the octothorp to the URL throw an error. This was because the `event` variable within the closeSlider/openSlider functions was not found. 

This patch passes those from the event listener down to the closeSlider/openSlider functions explicitly. I'm guessing some browsers (e.g. Chrome) have this available as a global while others do not.

Thanks for the library, it's quite useful!