Take over the 'multiple account model' feature to django-multiple-auth

[shimizukawa]

django-newauth will resolve two issues that are:

1. Django user model couldn't be replaced with developer defined model before Django-1.4.
2. provides separated user models for Admin users and Frontend users

For now, Django-1.4 supports user defined model. So, 'multiple account model' that provides frontend user accounts is needed.

To keep compatibility, django-newauth is kept as is in here.
I'll take over the 'multiple account model' feature to django-multiple-auth package.