First attempt at an S3 path_style flag

As discussed: https://github.com/fog/fog/issues/1631

The good news: This patch solves my issue.
The bad news: The fix completely depends on the - previously existing - side effects of Storage#signature on the params hash. So it fails during testing, since the method is then mocked. A 'proper' fix would probably involve removing all side-effects on the params[:path], injecting the setting during file/directory model creation and depending on their #url methods completely. Let me know what you think.