Django-sql-utils

Exists` needs to go one relation deeper if the final relation is a nullable foreign key.

Also added the pretty_print_sql utility.

pip install was failing when using the tar archive because the README is missing there.

Add a simpler API for the Exists Subquery Expression.

Fixes a bug in setting the output_field in Django 1.11

The first version, providing the simplified API for using Subquery to do aggregations.