fartscroll.js should not rewrite any existing onscroll event handler.

You can test new behavior using this code:
```JavaScript
        $(document).ready(function() {

            window.onscroll = function() { console.log('my handler') };

            // Ensure the page always has more scrolling
            $(document).scroll(function() {
                while ((window.pageYOffset + window.innerHeight) >= $(this).height() - (window.innerHeight / 2)) {
                    $('ul').append('<li>Testing.</li>');
                }
            }).scroll();

            fartscroll(800);
        });
```

With this patch included "my handler" will not be rewritten.