The process of choosing an image format has been cleaned up and
Processors' role in determining the format has been removed.
Previously, processors would return a tuple containing the modified
image and the format. Other parts of IK overrode PIL's Image.format
with the target format, although that had no effect on PIL and the fact
that it didn't throw an error was just lucky.
Removed the save and clear_cache methods from ImageModel (along with helpers).
Now, whenever an ImageSpec is contributed to a model, handlers are created for
the post_save and post_delete signals. The post_save handler does the work of
running the ImageSpec processors and caching the resulting file, while the
post_delete handler does the work cleaning up the cached files.
You're no longer restricted to just one, special-case admin thumbnail. Make as
many as you want by adding AdminThumbnailView properties to your model and
including them in your admin class's `list_display` tuple. You can also provide
a custom template. Note that (because this change introduces templates to
imagekit), imagekit is now required in INSTALLED_APPS.
Ideally we could get this stuff out of the model, but we'll have to look into
whether that's possible without making things really complicated.
By creating the Descriptor using contribute_to_class (instead of in
ImageModelBase's __init__), we take the first step towards eliminating the need
to extend ImageModel at all.
This works kind of like Django's models' _default_manager. If your specs don't
specify an image_field, and your IKOptions don't specify a default_image_field,
the first ImageField your model defines will be used.
