For development, ensure that local connections do not require a password. Locate your pg_hba.conf file by running SHOW hba_file; from the psql prompt (sudo -i -u postgres + psql after clean install). This should look something like /etc/postgresql/9.5/main/pg_hba.conf. We'll call the parent directory of pg_hba.conf the $POSTGRES_DATADIR. cd to $POSTGRES_DATADIR, and edit pg_hba.conf to trust local socket connections and local IP connections. Restart postgres - on Mac OS X, there may be restart scripts already in place with brew, if not use pg_ctl -D $POSTGRES_DATADIR restart.
Now, assuming the postgres database superuser is postgres, let's create the databases.
createdb -U postgres opencollective_dvl
createdb -U postgres opencollective_test
createuser -U postgres opencollective
psql -U postgres
> GRANT ALL PRIVILEGES ON DATABASE opencollective_dvl TO opencollective;
> GRANT ALL PRIVILEGES ON DATABASE opencollective_test TO opencollective;