New: create-story command

This PR adds a new 'create-story' command, which creates a Story.php file full of helpful comments to guide you through writing a new test.

    storyplayer create-story <path-to-story.php>

You can use the --based-on switch to tell the story which StoryTemplate(s) to base the story on:

    storyplayer create-story --based-on DataSift\QA\StoryTemplates\PickleTemplate <path-to-story.php>

By default, you can't overwrite an existing file, but the --force switch is available for those who know what they are doing.