postgres.pass setting enabled and some bug fixes for salt.module.postgres

The postgres.pass option mentioned in the documentation was actually defunct. I enabled it using the environment variable method.
This required a change to be made to the password argument for the postgres module.
I decided to rename the password of created or amended roles to rolepassword. However, I am happy to change that if anyone has a better suggestion.

Also a couple of bugs were fixed:
* the version check did not convert strings to integers and did not check correctly
* pg_authid is a protected table that holds passwords. I replaced it with pg_roles, the public table. (http://www.postgresql.org/docs/9.1/static/catalog-pg-authid.html)