Allow custom querysets.

Imagine having a simple MongoEngine document:

```
class User(Document):
    email = StringField(required=True)
    first_name = StringField(max_length=50)
    last_name = StringField(max_length=50)
    is_active = BooleanField(default=False)
```

Thanks to this pull request, if you'd like to show only active users in the admin, you'll be able to simple do:

```
class UserAdmin(model.ModelAdmin):
    list_display = ['email', 'first_name', 'last_name']

    def get_queryset():
        return self.model.objects.filter(is_active=True)
```

This approach works for all the backends.