Add detachEvent method for detaching scroll event

While this seems like a simple code refactor. I'm in a circumstance where I need to be able to temporarily disable the headroom event handling while I programmatically animate the scroll view when the user clicks a link.

Refactoring this allows me to use the headroom.detachEvent() and headroom.attachEvent() method as needed. 

I realize the attachEvent() is marked as @private, so if there is a better implementation you can think of, that would be great too.