PostgreSQL: Use `postgres:` scheme for DATABASE_URL & update plugin's README

According to [documentation](http://www.postgresql.org/docs/9.2/static/libpq-connect.html), both `postgres` and `postgresql` URL schemes are allowed. However, Heroku uses `postgres` schema, which seems to be de facto standard (at least in Ruby world); [Sequel](http://sequel.jeremyevans.net/), for example, won't accept `postgresql:` schema as a connection string at all. Furthermore the variable is set after the application config, so it cannot be overriden by custom configuration.

I've also added information about the plugin into its README and removed MariaDB references.