INTPYTHON-348 add QuerySet.raw_aggregate()

[aclark4life]

Via @timgraham

I don't think Django provides a hook that will let us override or customize the behavior of QuerySet.raw() , so I think we'll have to write a custom manager that developers could use on their models that will override Queryset.raw() (or probably it's better name it something different like raw_mql() ). The link between Manager and QuerySet is here: https://github.com/django/django/blob/438fc42ac667653488200578a47e59f6608f2b0b/django/db/models/manager.py#L176-L177
So I think we need a subclass of QuerySet with a raw_mql() method and then something like:

class MongoManager(BaseManager.from_queryset(MongoQuerySet)):
    pass

This could live in django_mongodb/manager.py

Then usage on the model would be:

class Model(models.Model):
    objects = MongoManager()