Expose model with QuerySet.model which works with Django QuerySet

[debugger22]

In [6]: from django.contrib.auth.models import User

In [7]: User.objects.all().model
Out[7]: django.contrib.auth.models.User

In [8]: type(User.objects.all())
Out[8]: django.db.models.query.QuerySet

In [9]: type(MongoUser.objects.all())
Out[9]: mongoengine.queryset.queryset.QuerySet

In [10]: MongoUser.objects.all().model
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-11-846b78ff429d> in <module>()
----> 1 MongoUser.objects.all().model

AttributeError: 'QuerySet' object has no attribute 'model'

Here MongoUser is a Document class and User is a simple user model from django.contrib.auth.
This is necessary to make it work with dhango-rest-framework library.

Thanks for all the great work!

[mmellison]

Label and close as part of Django split. See #973.