SimpleEmailConfirmationUserMixin is not really optional

documentation says about SimpleEmailConfirmationUserMixin:
> Note: you don't strictly have to do this final step

but I fail to create user when SIMPLE_EMAIL_CONFIRMATION_AUTO_ADD is True:
```
  File ".../lib/python2.7/site-packages/simple_email_confirmation/models.py", line 207, in auto_add
    email = user.get_primary_email()
AttributeError: 'User' object has no attribute 'get_primary_email'
```


This pull request fixes the problem but it's not DRY. Maybe it's better to create some module level methods like this:
```
def get_primary_email(user):
    primary_email_field_name = getattr(user, 'primary_email_field_name', 'email')
    return getattr(user, primary_email_field_name)
```

and then use them in mixin and in signal handler