Remove dependency on 'username' field

[atleta]

Hi,

I've just looked into the code and found that there is a wired in dependency on the username field of the user model. However, it may not be there. You should use get_user_model().USERNAME_FIELD to refer to it, but for example for creating a user, you don't even have to name it. Just pass in a **kwargs .

[lambdalisue]

Hum... Yes you are right. However, it would be tough work to remove the dependency so I'd like to try it when more users had problem with that dependency (https://github.com/lambdalisue/django-inspectional-registration/search?p=1&q=username&utf8=%E2%9C%93)

If you really need to remove that from django-inspectional-registration, please consider to send me a pull request ;-)