jQuery.fn.center = function()
	{
		var w = $(window);
		this.css("position","absolute");
		this.css("top",(w.height()-this.height())/2+w.scrollTop() + "px");
		this.css("left",(w.width()-this.width())/2+w.scrollLeft() + "px");
		return this;
	}

jQuery.fn.hide_show = function()
	{
		if (this.css("display") == 'none'){
			this.show();
		} else {
			this.hide();
		}
		return this;
	}