Adds image-view to render BufferedImages.

Straw man implementation of image-view. To test:

``` clojure
(import javax.imageio.ImageIO)
(require '[clojure.java.io :as io])
(def i (ImageIO/read (io/file "screenshot.png")))
(use 'gorilla-repl.image)
(image-view i)
```

Options supported by `image-view` are:

`:alt` : alt text for the image tag. Default "".
`:type` : output format of the image ("bmp", "gif", "png", "jpg"). Default "png".
`:width` : width of the output image. Default input image width.
`:height` : height of the output image. Default input image height.

Aspect ratio is preserved if only one of the size options is specified.