Add support for PostgreSQL PGDG APT repository

PostgreSQL Global Development Group APT repository maintains packages
for different PostgreSQL versions as well as additional software. More
information: https://wiki.postgresql.org/wiki/Apt

PGDG APT repository can be enabled by setting 'postgresql_pgdg' boolean
variable to True.
Add support for PostgreSQL 9.3 in 'postgresql'

To install PostgreSQL 9.3, you need to enable PGDG APT repository and
change 'postgresql_version' variable to "9.3".
Merge pull request #28 from drybjed/role/postgresql

Updates in 'postgresql' role