Update skrollr.menu.js

I want know when the hash change, so i propose these change.

and use it like that:
```
            // Init Skrollr
            s = skrollr.init();
            skrollr.menu.init();

            s.onHashChange = function(hash) {
                    jQuery.trigger('hashchange'); // yeah !!! i can catch hashchange event !!! 
            };           
```