The Format processor was really a special case and didn't do any
processing at all. Instead, ImageSpec just knew to look for it and
responded accordingly. Therefore, it's been replaced with a `format`
property on ImageSpec. This warranted a deeper look at how the format
and extension were being deduced (when not explicitly provided); the
results are documented in-code, though the goal was "no surprises."
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.
Removed the cache_dir, cache_filename_fields and cache_filename_format
properties of IKOptions. While these were very powerful, I felt that it was
unnecessarily confusing to have two properties (cache_dir and
cache_filename_format) that determine the filename. The new cache_to property is
modeled after ImageField's upload_to and behaves almost identically (the only
exception being that a callable value receives different arguments). In
addition, I felt that the interpolation of model properties provided by
cache_filename_fields, though useful, would be better handled by a utility
function outside of this library.
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.
