Nginx X-Accel Support

I've been experimenting with speeding up image transfers.  This patch uses nginx's X-Accel feature to offload file streaming to nginx.  I've noticed  ~25% speed increase with this feature using `wget`.

To use this, add the following to `config.yml`:

```
nginx_x_accel_redirect: /registry
```

And setup the `/registry` location in nginx to point to the root of the registry:

```
server {
    [...]

    location /registry/ {
      internal;
      alias   /srv/registry/;  # note the trailing slash
    }

    location / {
        proxy_pass  http://docker_registry;
    }
}
```

Please let me know your thoughts and I can work on tests.

Also, this version puts the logic in `images.py`, but I think it's better suited in the `LocalStorage` class.  Moving it there would require `Storage` classes to have some type of mechanism to modify headers; this can be done with an additional `get_headers()` call.

Thoughts?