Added support for sslmode to PostgreSQL connector.

This change makes it specify sslmode when connecting to PostgreSQL. Of course this requires some configuration of PostgreSQL to make it work. I've tested this with the database config value 'require' to make SSL connection mandatory. Other values depends on the pgsql library in use. 

As of verson 9.3 there exists a handfull of sslmodes:
http://www.postgresql.org/docs/9.3/static/libpq-ssl.html