Carry over Exif camera metadata.

I'm already getting getting requests to allow display of camera information such a camera model, lens setting, etc. Add a way to carry this information over when processing photos. The best way would probably be just to store that information in some sort of Exif namespace (which could be an URF sub-namespace such as exif-make), and then update the Exif addition code to write it if present. That way it becomes part of the official image artifact metadata, and can even be processed by the site and included via Guise Mesh interpolation.