// jQuery extension function for centering div on screen. Adopted from 
// http://stackoverflow.com/questions/210717/what-is-the-best-way-to-center-a-div-on-the-screen-using-jquery
jQuery.fn.center = function () {
    this.css("position","absolute");
    var top = ( $(window).height() - this.height() ) / 2+$(window).scrollTop();
    if (top < 0) top = 0;
    this.css("top", top + "px");
    this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");
    return this;
};