Allow selection of schema for Postgres datastore

[jusw85]

Postgres datastore uses 'public' schema implicitly.
Allow configuration to select the schema used.

Implementation wise, probably prefix all SQL/DDL statements with
SET LOCAL search_path TO <schema_name>;

[subnetmarco]

By closing #1198 Kong will now uses the default search_path whose value is '"$user", public' (http://www.postgresql.org/docs/9.1/static/runtime-config-client.html).

An additional update on the Postgres client is required to make this value configurable. Assigning this to @thibaultcha since he built the Postgres driver.

[bungle]

Schema can be selected:
#4167

So closing this.