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:

With this change in config.example we force the developer to configure his smtp settings for authentication if required or desired and if he don't need authentication, for example if he is using mailcatcher, the application won't crash.