Allow PolymorphicQuerySet.instance_of to accept strings.

The interface is similar to passing strings into ForeignKey and helps resolve circular dependencies.

I can add tests and docs later but I wanted to get feedback on the approach first. I had originally put it in [`_create_model_filter_Q`](https://github.com/chrisglass/django_polymorphic/blob/master/polymorphic/query_translate.py#L213-L248) but you lose the "app label" awareness from the instance that allows you to pass in `ModelName` (as opposed to `app_label.ModelName`).