Added Nginx X-Accel support.

When the configuration contains the directive below, sending the file to the
client is handed off to nginx [1], which frees the Python process from having
to stream the file:

```
nginx_x_accel_redirect: /registry
```

For this to work, the /registry location needs to be configured in nginx:

```
server {
    [...]

    location /registry/ {
      internal;
      alias   /srv/registry/;  # note the trailing slash
    }

    location / {
        proxy_pass  http://docker_registry;
    }
}
```

[1] http://wiki.nginx.org/XSendfile
Added check for LocalStorage.

nginx_x_accel_redirect cannot be used unless storage is local.
Fixed PEP8 test warnings and errors
Updated variable name

Instead of `info`, make the variable related to its purpose.
Added tests
Merge pull request #77 from rca/nginx_x_accel_redirect

Nginx X-Accel Support