Resolve a major issue with storagefile.read()

For the longest time thumbnail libraries have had issues with `django-cumulus`. 

Previously `self.file` was being to a pyrax storage object in the `__init__` super call, however the pyrax file subclass does not implement the readline method. As a result Django would fall into an infinite loop when trying to read the file, because django expects the readline method. 

This PR is not a perfect fix, and I can't prove that gzip read is working correctly, but the non-gzip files are working as they should and as a result thumbnailing is finally working. 

@ferrix I'm out of my element here, so any insight into what is going on here would be greatly appreciated.

Note #129 and https://github.com/matthewwithanm/django-imagekit/issues/263