Include support for serving gzip version of static file if present on disk

As this module is a dependency used by dj-static https://github.com/kennethreitz/dj-static which in turn is referenced and recommended by Heroku https://devcenter.heroku.com/articles/django-assets to serve static content for django applications coupled with the conversation here: https://discussion.heroku.com/t/django-static-file-gzip-compression-not-working/321 I found it necessary to make these changes to server pre-gzipped content from my wsgi django app on heroku. I realise that updating this repository is a better aproach than simply overriding the class used in dj-static as I had suggested.

maybe little simpler:

```python
    prezipped = ('gzip' in environ['HTTP_ACCEPT_ENCODING']
                 and path.exists(full_path + '.gz'))
    if prezipped:
        full_path += '.gz'
```