New sort method (date taken) for Pictures

This dev adds a new way to sort Pictures - by "Date Taken". It uses the EXIF timestamps to do the sorting. This allows viewing photos from multiple cameras in a logical sequence irrespective of the file modified date, file names or any of the existing sort methods.

The dev also allows python plugins to use the new sort method.

The discussion thread linked to this is http://forum.xbmc.org/showthread.php?tid=156942