Create the settings.php file if missing.

Use case: wipe data from test installation in order to start over quickly... 1. drop/empty the db 2. delete ./files and its contents 3. delete settings.php 4. visit site to re-start the installation process

As it is now, the installer complains about a missing settings.php:

So I had to extract a fresh copy of the file in order to carry on. Can we ship a default-settings.php file and have the installer create a copy of it instead of complaining in this case?