Unbind/destroy

Added a `destroy` method to unbind/reset plugin bindings and side effects. Binding/destroying does not happen when "fixedsticky" is supported. Initialising (like before) and destroying works like this:

    $(el).fixedsticky();
    $(el).fixedsticky('destroy');

Additionally removed the plugin from global namespace as I currently don’t see any reason to expose. Hope your fine with that :)