Previously, ImageSpecFile instances were retrieved (for saving and deleting,
among other possibilities) by iterating over the model instance's attributes.
This change adds ImageSpecFile names to a list (spec_file_names) on an
imagekit meta object on the model (_ik), making later retrieval much cheaper
and more straightforward.
Replaced calls to Image.open with an open_image utility function.
The open_image utility calls Image.open, but then wraps the opened Image's
copy method with a version that preserves EXIF data. This allows an
ImageSpec to copy the original image, yet still provide all the original
image's exif data to the processor pipeline.
