Cloudinary storage

This adds a storage backend for Cloudinary images and replaces `CloudinaryField` with a subclass of `ImageField` for more seamless integration with Django.

I suspect most users uploading images to remote servers are currently using a similar approach (like [django-storages](http://code.larlet.fr/django-storages/)) so this makes for a nice drop-in replacement.

Test cases are still needed, but I'm curious to see if there's any interest in this approach. I'm also interested in expanding this to allow tags and eager transformations to be specified for fields.