Feature: Implement sprite image. (Issue #466)
Using https://github.com/am11/RectanglePacker
Usage:

// Using System.Linq, Mapper
// imageFileCollection is IEnumerable<string>
var rectangles =
       imageFileCollection.Select(path =>
           {
           var image = Image.FromFile(path);
           return new ImageInfo(
                (int)image.Width,
                (int)image.Height,
                path);
           }
       );

// destinationSpriteFile is string
// (desired/path/to/sprite.png)
new SpriteRunner(rectangles)
  .GenerateSpriteWithMaps(destinationSpriteFile);

// Now destinationSpriteFile (.png) has image,
// and .sprite is JSON-based map.
// Call optimizer on destinationSpriteFile