Adding ability to unbind scroll handler and destroy plugin object

[snvl1993]

Hi!

As i can see there was no ways to destroy plugin. So if somebody uses, for instance turbolinks (issue #5), he faces with problem, when infinity pages from old page still working in the new one.

So i think, it can be useful to get ability to destroy plugin.

Example of usage:

# Bind plugin on page load
$(document).on "turbolinks:load", ->
  $(someElement).infinitePages settings

# Destroy plugin when user leaves the page
$(document).on 'turbolinks:visit', ->
  $(someElement).infinitePages 'destroy'