Simplify use of Docker to run postgres for tests

[nexdrew]

Ran into a problem with PostGIS again when recently trying to use docker-compose to stand up a postgres container for running tests locally.

E: Version '2.2.2+dfsg-1.pgdg80+1' for 'postgresql-9.4-postgis-2.2' was not found
E: Version '2.2.2+dfsg-1.pgdg80+1' for 'postgis' was not found
ERROR: Service 'postgres' failed to build: The command '/bin/sh -c apt-get update       && apt-get install -y --no-install-recommends            postgresql-$PG_MAJOR-postgis-$POSTGIS_MAJOR=$POSTGIS_VERSION            postgis=$POSTGIS_VERSION       && rm -rf /var/lib/apt/lists/*' returned a non-zero code: 100

This got me interested in finding the simplest possible setup for a postgres container. Since oauth2-server-pg actually doesn't need PostGIS, I was able to completely replace the "docker-compose with custom image" approach with this simple docker run command:

docker run --name oauth2serverpg_postgres_1 -e POSTGRES_DB=oauth2_server -p 5432:5432/tcp -d postgres:9.4 postgres

The standard postgres image will use the POSTGRES_DB env var for the default database, so no need to create the oauth2_server db on/after startup.

So I created an npm run script for this and wrapped it in a conditional pretest hook, which automates the setup and tear down of the postgres docker container locally, but doesn't get in the way when run by Travis. This means all you need to run tests locally is to have docker installed and then run npm t, and it will use a throw-away container for postgres for the duration of the tests.

Also added these handy scripts for convenience:

• npm run pg-test: start up the dockerized postgres db and run migrations
• npm run psql: login to the running postgres container to run some manual queries
• npm run pg-test-down: destroy the running postgres container

[coveralls]

Coverage remained the same at 89.583% when pulling be1d5bf on nexdrew/simplify-docker into d6ee89c on master.

[bcoe]

I'm comfortable with giving inlining the docker logic a shot.