Use official Postgresql package repositories

This patch resolves issue #187 of using old 9.1 postgres on newest Ubuntu 13.04 by using official Postgres repos for installation. I've added a fallback to install postgresql from OS default repos when there is no matching official repo was found. 

I've also added postgresql-contrib since it has too many great extensions that we almost always need in development. Foreign data wrappers, pg_trgm or unaccent to name a few. Also it adds pgadmin3 which is an indispensable tool for tuning query plans and advanced administration even if the GUI takes a while to get used to.

What do you think?