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