Django 1.5 Custom User Model Compatibility

I've made some little changes to make the app compatible with Django 1.5 user model customization features. 
In a few words, instead of using ``django.contrib.auth.models.User``, I'm calling a new defined function: ``django_facebook.utils.get_user_model()``.
It wraps ``django.contrib.auth.get_user_model`` around, and falls back to the default User model whether it's not defined.