Introducing the setheight option

Setting it to false will prevent unslider from setting CSS height on the slides or slider-element thus honoring any previously set CSS

I had this problem when using it on a site where the backend resizes images directly on request and because of this the image had not arrived before unslider decided to set the height. I know I could have used imagesloaded or some similar plugin to postpone the unslider start, but I'd rather waste less CPU-cycles :)