Adds NW build scripts.

[danamouk]

This pull request adds PostgreSQL build scripts and documentation for the Northwestern ICU (NWICU) dataset on PhysioNet.

The following files are added under nw/buildnw/postgres/:

README.md – setup and usage instructions

create.sql – schema creation script

load_gz.sql – load compressed CSV data

constraint.sql – primary/foreign key constraints

index.sql – indexes for performance

validate.sql – validation checks

postgres_env.sh – environment variable helper