defined type for creating database schemas

This defined type helps create database schemas, and assign them to an
`owner`. It is closely modeled after Postgresql::Server::Tablespace.
It uses PostgreSQL's builtin IF NOT EXISTS to guarantee idempotency.
(>= 9.3, else it checks pg_namespace).

n.b.: This defined type *requires* that a `db` is passed. This is a
concious design decision, since we find it rather useless to create such
schemas in the default `postgres` database, and if *were* useful, one
can always "over-specify".

This addresses MODULES-1098.