New: warn when stories do not end in Story.php

If your story file does not end in 'Story.php', we now output a warning to the output mechanisms.

Why do we need this warning?  

* The command `storyplayer <folder>` will only play files that end in 'Story.php'.
* I've noticed that it's very easy for people to forget / not realise that their stories should end in 'Story.php'