There has been some refactoring going on in the pinry.pins.models module.
The upload_to code has been refactored into its own function, images
have been moved to their own models - otherwise the number of fields
in the Pin model would skyrocket. Also ModelManagers have been written
to move image fetching and generating outside of models.
Tastypie requires that we define a list of fields that can be used
for filtering; add the "published" to this list so we can query pinry
for the list of images created after some date.
