Support reverse proxies for IP-based permissions

Fix for #111, allows running LocalShop behind a reverse proxy such as Nginx or Apache and having IP-based permission checks work.

Adds a new setting `LOCALSHOP_USE_PROXIED_IP`, which when True enables using the standard `X-Forwarded-For` header rather than `request.REMOTE_ADDR` for IP-based permission checks. Default is False for obvious security reasons.