Add PostgreSQL database support for gerritdb

This adds a dependency at the operating system level for:

    You need to install postgresql-server-dev-X.Y for building a
    server-side extension or libpq-dev for building a client-side
    application.

Change-Id: I9e17d2e11c2f73f12730bf5d7f27b31cedd7cbd9
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>
Merge "Add PostgreSQL database support for gerritdb"