pg_user recipe was not idempotent. Updated sql for testing the existence of a user to  query against pg_roles instead of pg_user. A user created with NOLOGIN will not have an entry in pg_user table, thus subsequent runs would fail if a user was created with NOLOGIN
Merge pull request #57 from stianselland/fixing_duplicate_role_check

pg_user recipe was not idempotent. Updated sql for testing the existence...