Add function to get scroll item's position

Usage example : 
     
```   
skrollr.menu.init(app.skrollr.instance, {
                    animate: true,
                    easing: 'sqrt',
                    duration: function(currentTop, targetTop){
                        return 1000;
                    },
                    itemPosFunction: function(){
                        var href = $(this).attr('href');
                        if(app.isMobile())
                            return app.skrollr.instance.getScrollTop() + $(href).offset().top - $(href).height() - 40;
                        
                        return $(href).offset().top - $(href).height() - 40;
                    }
                });
```