Direct s3 download via redirect

This adds a method to the storage strategy allowing it to provide a URL to which client can be redirected.
For S3 backed storage this offloads I/O to S3 and makes the download go even faster.

For people who don't want to put their registry behind a CDN and persist errors into that CDN (as with cloudflare) this is a decent solution. We could make it optional in config.

I also added S3 region to config. I've always had problems whenever I didn't use the right S3 end-point with boto.
But it could just be who is paranoid, see boto/boto/issues/621

(Credit where credit is due, the redirect idea was prototyped by [James Lal](https://github.com/lightsofapollo/docker-registry-caching), this is just a cleaner implementation).