Add SubfieldBase to call to_python

Currently `to_python` is not called when getting the value from the database
`to_python` is called either when assigning a value to the model or when retrieving a value from it. It should be either be able to return the string assigned or to decrypt it. This behavior (from django) is not ideal but will change in the future see https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#converting-values-to-python-objects

Here I haven't found a better way to know when I need to return the value assigned to the model and when I'm retrieving the value from the database.