Remove models to tests.models
Add django_base64field.tests as an app to get covered ;)
Add CustomReceiver model

Passing custom receiver to generate `youyouid` with a custom receiver.
Add little bit comments for Planet model
Move tests to separate module
Mode attr default change on Base64Field
Add test_field_with_custom_receiver_method

Should use a different receiver method for generating a custom
base64field. For this test `CustomReceiver` model will be used which
it will use `django_baset64field.tests.receivers:custom_receiver` as a
custom receiver for `Base64field`.
Add custom_receiver

Set `youyouid` field with custom uuid.uuid4() ;)
python3 support ;)