Overview
-------------
Adds a "bottom" event that will be triggered when the user has scrolled
to the bottom or within proximity to the bottom of an element.

Syntax
-------------
$(element).bottom(options);

Options
-------------
proximity (optional - default: 0):How close to the scrollbar is to the bottom of the element before triggering a load.  Use proximity: 0 for absolute bottom.
	Proximity = (total scroll height - current scroll position) / total scroll height.

Example Usage
-------------
// we start with 1 page of data
var page = 1;
$(document).ready(function() {
    $(window).bottom();
    $(window).bind("bottom", function() {
        var obj = $(this);
        // since this ajax call might take a while
        if (!obj.data("loading")) {
            obj.data("loading", true);
            $("#news_list").append("<li>Loading...</li>");
            $.getJSON("/news/list?page=" + (++page), function(data) {
                // remove the loading text
                $("#news_list li:last").remove();
                for (var i = 0; i < data.news.length; i++) {
                    $("#news_list").append("&lt;li&gt;" + data.news[i].headline + "&lt;/li&gt;");
                }
                // now that the ajax call is done, we can re-enable this
                obj.data("loading", false);
            });
        }
    });
});