Move `/S3.py` to `/storages/utils/S3.py`

This makes it possible to directly import the `S3.py` utility module
when django-storages is installed as pip-package, e.g.
`from storages.utils.S3 import CallingFormat`.
Merge pull request #10 from malthejorgensen/py3-utils-dir

Move `/S3.py` to `/storages/utils/S3.py` (resolves issue #9)