Make picture tags complete and consistent

I posted a thread explaining what this commit is for. 

http://forum.xbmc.org/showthread.php?tid=158723

It makes all of the available picture info accessible for skins or addons to use to overlay on pictures when they are selected from the picture list, or from slideshows when they are displaying a picture. Or just for addons to use as needed.

This commit makes all of the picture metadata retrieved from a file available as both slideshow.* INFO variables and ListItem.Picture* INFO variables.

There is an additional fix in this commit.  The IPTC code 0x0A does not represent a Copyright code - there is no such thing as a copyright code.  The IPTC code 0x0A is the code for IPTC.Urgency.  I fixed this in the libexif source.  I also changed the variables for copyright code to urgency in the XBMC code. Typically, urgency is used for rating pictures.  A variety of image editing programs store rating in the urgency field. So we now have a way to make picture rating available in XBMC.