Adding tests for Postgres schema generation

[sashman]

Using pagila, sql dump added to repo, as an example. The tests rely on an available database with the sql dump imported. I suggests the following docker command:

docker run --name plsm-postgres-test -e POSTGRES_PASSWORD=postgres -p 5432:5432 -v `pwd`/test/support/schemas/postgres:/docker-entrypoint-initdb.d --rm postgres

Once this is up and running, you can use:

mix test

So far only one test has been implemented since plsm doesn't generate compliable code from this :(. #101 definitely fixes an issue.

What are your thoughts @jhartwell? I think we need to tackle #53 step by step.

[sashman]

@jhartwell rebased, just bear in mind these are first steps towards getting a full test suite, as it stands running mix test will very likely fail