Django-field-cryptography

- Add `metaclass` for `SubfieldBase` to call `to_python` when getting value
back from the database.

- Add tests
- Replace [SimpleAES](https://github.com/nvie/SimpleAES) with
[cryptography](https://github.com/pyca/cryptography)
- Add .travis.yml
- Update package description
- Rename project from `django-simple-aes-field` to `django-field-cryptography`
making `django-simple-aes-field` deprecated.