Register user with form's save method, whenever possible

Hey,

In a nutshell, as I added to the `CHANGELOG`:

* Feature: Added settings' options that allows to exlude the default auth urls (`INCLUDE_AUTH_URLS`) and register url (`INCLUDE_REGISTER_URL`).

* Feature: Added support to dynamically import any chosen registration form using the settings option `REGISTRATION_FORM`.

* Enhancement: Make `RegistrationForm` a subclass of Django's `UserCreationForm`

* Enhancement: Use registration form `save` method to create user instance, whenever form is a subclass of Django's `ModelForm`.

Basically I wanted to use your useful app in a project I'm working on, but with a few twists:

* I didn't want to use the `register/` url, because I have two distinct profiles each one with a different register form and view.

* I wanted to use a registration form of my own, which is a subclass of `ModelForm`, with logic in the `save` method, associated with a custom user with no username field.


I've run the tests and everything is working. I've also updated the docs, version number and changelog.

Hope you approve my work.
If you have any doubt, please contact me and I'll try to explain things better.

DLM