S3 Repository: Update the bucket and support compression

To optimize download time, images are now stored compressed. Go client
uncompressed them on its own, but for clarity the images are stored with
a gz extension that need to be removed when storing locally.

In addition a new bucket name is in use:
http://osv.capstan.s3.amazonaws.com/

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>