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
Don't set a secret_key in the Dockerfile

Setting the key in the `Dockerfile` means everyone that pulls the
image from the public index gets the same key. This moves the key
generation to the `setup-configs.sh` script.
Document configuration options
Provide sample config
Document Apache config
Fix email_exception documentation
Document secret_key
Document that the secret key is generated automatically if left blank
Document the index_endpoint
Final doctweaks
Line-wrap at 80