Add resetPage method and 'extra' option

I'm using this in production where I have a sort button that changes the sorting of the list. Therefore I needed a resetPage method and a method that attaches some information to the request for a new page (e.g. which sort or filter is applied).

Usage:

    $("#items").infiniteScroll.resetPage();
    $("#items").infiniteScroll.setExtra({sort: 'popularity'});

Can also be set at the beginning and the data will be sent along:

    $("#items").infiniteScroll({
        itemSelector: '.widget',
        extra: {some data here},
        ...
    });

Also I added a cache for data in the last row that is encountered first. So one can do `<div class="widget" data-id="123">` for an item and the infinite scroll will attach `id=123` to every request and the backend can better figure out pagination stuff if needed.