jQuery.fn.animateAuto = function(prop, speed, callback){
    var elem, height, width;
    return this.each(function(i, el){
        el = jQuery(el), elem = el.clone().css({"height":"auto","width":"auto"}).appendTo("body");
        height = elem.css("height"),
        width = elem.css("width"),
        elem.remove();

        if(prop === "height")
            el.animate({"height":height}, speed, callback);
        else if(prop === "width")
            el.animate({"width":width}, speed, callback);
        else if(prop === "both")
            el.animate({"width":width,"height":height}, speed, callback);
    });
}