Meet requirements for referencing custom User model in Django 1.5 and 1....

Hello!

I have one more pull request for you project. This time I can't go forward because my custom User model requires explicit referencing. According to [docs](https://docs.djangoproject.com/en/1.7/topics/auth/customizing/#referencing-the-user-model) in order to reference User model as foreign key or signal sender we have to use AUTH_USER_MODEL setting.

Also, I removed strings

    try:
        from django.contrib.auth import get_user_model
        User = get_user_model()
    except ImportError:
        from django.contrib.auth.models import User

from both modules as User variable isn't used there anymore.
Connecting to model signals lazily allowed [only since django 1.7](https://docs.djangoproject.com/en/1.7/ref/signals/#module-django.db.models.signals) so we should check django version in this place.