Per-tile images

Hi! In my current project, I'm planning on adding a build step that puts multiple tile images together into one tileset texture. Tiled supports this feature since version 0.9 according to the spec. This is done by omitting the <image> element from <tileset> and providing one for each <tile> instead. I'd like to use this library for loading tilesets and maps, but this is the one thing missing before I can do so.

As far as I know, this would only require adding a field to TiledTile. Something like

/// <summary>
/// Defines the individual tile's image.
/// </summary>
[XmlElement("image")]
public TiledImage Image;

/// <summary>
/// Nothing to see here. Used for serialization.
/// </summary>
/// <returns></returns>
public bool ShouldSerializeImage()
{
    return Image != null;
}

I can open a pull request when I get home if you're interested in this.