attempting to fix the seek issue using stringio

This is actually the root of the issue that I was trying to fix with #56. Turns out it wasn't the django-storages version, but rather a problem with calling seek() on an HttpResponse object (generated by the requests.get call). What I did was read in the raw response data and passed that to StringIO, which seems to resolve the issue. I can now save files using the django-storages S3boto backend.

It would be great if someone could test its effectiveness with other storages.