Upgrade to PostgreSQL 9.3 with extension whitelisting

Changes:
 * Use stackbrew/ubuntu:12.04 as base image
 * Use the PGDG (PostgreSQL Global Development Group) apt repository
 * Upgrade to PostgreSQL 9.3
 * Enable extension whitelisting (like Heroku) to allow enabling
   extensions by normal users
 * Add a slightly tuned postgresql.conf (optimized for 512MB/50 conns)
 * Use default storage locations, add Volumes for these