Added PostgreSQL support, including:

* PostfixAdmin::MySQL library renamed to PostfixAdmin::DB.
* PostfixAdmin HTML error parsing improved.
* required packages installation refactored using attributes.
* Fixed config.local.php group value without apache.
* kitchen.yml: added hostname and forwarded ports.
* All integration tests fixed to pass.
* TODO: added some tasks.

Based on @anveo work: https://github.com/anveo/postfixadmin-cookbook/commits/postgresql-support