Support Postgresql Schema for PostGIS DB

A Postgres database contains one or more named [schemas](https://www.postgresql.org/docs/9.6/static/ddl-schemas.html), which in turn contain tables. Default is the `public` schema. Especially with PostGIS the use of schemas is recommended practice (all PostGIS functions and ref to PostGIS libs reside in the `public` schema, think of issues when making native dumps). The current GHC implementation does not support PG schemas yet.

PG Schemas with SQLAlchemy requires special handling, i.e. there is (AFAIK) no provision in the DB URI, provided in GHC via the `SQLALCHEMY_DATABASE_URI` config setting. See for example [here](https://stackoverflow.com/questions/9298296/sqlalchemy-support-of-postgres-schemas).

Possibly we can extend the `SQLALCHEMY_DATABASE_URI` with a schema or have a schema variable.