Implementing stale while revalidate

[andreapernici]

I would suggest an enhancement for the image delivery. It makes sense a lot to setup the SWR nginx stuff for images.

        proxy_cache_use_stale updating error timeout http_500 http_502 http_503 http_504;
        proxy_cache_background_update on;

[noogen]

Added to default config, since it's a config change, I've rolled up into issue #21 update too.