[DBAL-702] Add sslmode connection option to pdo_pgsql driver

Allows specifying whether or with what priority a secure SSL TCP/IP connection will be negotiated with the server in `pdo_pgsql` driver as requested in [DBAL-702](http://www.doctrine-project.org/jira/browse/DBAL-702).
See: http://www.php.net/manual/en/ref.pdo-pgsql.connection.php#112685