Add GZIP_CACHE_OVERWRITE option to overwrite original files

This option is useful when hosting static content on a service such as
Amazon S3. Setting to True will overwrite the original file with
the gzip-compressed file. In order to correctly serve the file, the
Content Encoding must be set to gzip by the hosting service.
Merge pull request #298 from jkrshw/feature/gzip-overwrite

Add GZIP_CACHE_OVERWRITE option to overwrite original files