Make directories automatically and don't require '.yml' on keys in translations.yml file

I've added a call to FileUtils.mkdir_p on Rails, for autmatically creating config/locales/*locale* dirs.
And removed the comment for that on the Readme, as it's no longer necessary.

Also, the keys in the `config/translation.yml` now can be without the .yml extension:
```yaml
files:
  navigation: "https://docs.google.com/spreadsheet/pub?key=ab43...34f3&single=true&gid=0&output=csv"
  forms: "https://docs.google.com/spreadsheet/pub?key=0Ap...XveWc&single=true&gid=0&output=csv"
  ... etc ...
```

And the gem will automatically add the .yml when creating the translations files.
Anyway, this is optional and documentation hasn't been changed. But I believe it leds to a cleaner configuration.