removing smtp_settings -> user_name from config.example default

When :user_name: is configured it's required to configure :password: or the application throws an error when trying to send an email: `SMTP-AUTH requested but missing secret phrase`

Because of this, I removed :user_name: as default in config.example and I added a new key to configure default :from used by mailer, this key is :sender_address:. This was needed because default :from was configured by :user_name:, but it's not correct since we can have a smtp configuration without authentication and in this case we don't have a :password: