jQuery.fn.centerOn = function (left, top) {
	return this.each(function () {
		var $self = jQuery(this);
		var width = $self.width();
		var height = $self.height();
		$self.css({
			marginTop: -height/2,
			marginLeft:  -width/2,
			left: left,
			top: top
		});
	});
};