Add functions to return sprite image dimensions

Hi guys.

Last week I created these functions, and I'd like to see this in the Compass Core

I case of use
```scss
$my-sprite-map: sprite-map("my-folder/*.png");

.my-element {
    $height: sprite-height($my-sprite-map, "some-amazing-image");
    position: absolute;
    top: -($height / 2);
    left: 0;

    background: sprite($my-sprite-map, "some-amazing-image");
    width: sprite-width($my-sprite-map, "some-amazing-image") - 10px;
    height: $height - 10px;
}
```
**Note**: I don't have tested this snippets... sorry for some possible stupid error.

I don't know how I can test this in ruby, because I don't know ruby :/ but is just it :)

Att