pycloudinary does not implement Django's storage API

The `CloudinaryField` is not a `FileField` which makes it harder to upload images with Django.
Creating a custom storage backend is not that hard.
See the [documentation](https://docs.djangoproject.com/en/1.7/howto/custom-file-storage/) for details.

Start with Django's custom file storage documentation linked in the issue, then inspect how CloudinaryField currently integrates with Django. The work is complete when CloudinaryField supports Django's storage API and image uploads can use it as a FileField-compatible storage path; the payload names no repository files or tests.

索引モデルが issue の本文から書いたものです。