Expand imfinfo to read Exif tags and GPS information. * __magick_read__.cc (is_valid_exif, fill_exif, fill_exif_ints, fill_exif_floats): New functions to parse the strings returned from GraphicsMagick's attribute() and get actual value in the Exif tag. (__magick_finfo): implement reading of Exif tags. New fields in the base struct being returned: Software, Make, Model, DateTime, ImageDescription, Artist, Copyright, DigitalCamera, and GPSInfo. The last two are structs in their own that contain the rest of values from Exif (a rather long list of possible values). Move obtaining DisposalMethod outside of the main loop since it is only done for GIF files. * imfinfo.m: document new fields on output.