I never liked that the "AdminThumbnailView" was supposed to be defined
on the model, but never looked into it.
This commit puts the definition back where it belongs: in the admin.
Instead of requiring you to add a field (with view logic) to your
model, you now just add a property to your admin class and specify
that property in the `list_display` list.
Crop doesn't necessarily imply the any scaling is taking place. Several
ideas were discussed, from renaming Crop to combining both processors
into a single Resize processor (as they were in the original IK), but
those solutions were felt to either precluded future extension
(alternative resize modes) or make the API too verbose.
