Additional thumbnail attributes

[stephenwf]

When parsing a thumbnail it might be useful for clients to know if the thumbnail originates from an Image Server and if the aspect ratio of the thumbnail matches the canvas. These two pieces of information would allow the client to make a decision if it wants to use the canvas a small representation of the canvas (and not just a navigation element) in a larger deep zoom interface. (similar to a LOD in 3D).

This could be returned as some extra fields on the thumbnail helper.