Adds base support for IPs over load balancers / proxies using X-FORWARDED-FOR header.

If you're using throttle middleware under an Nginx configuration, you'll ended up blocking almost every user by default, unless you create your own function to "check" who is who. This commit tries to solve at least the minor problem detecting if a X-FORWARDED-FOR header comes with the request, and if so, returns the IP from that header instead of the detected by Go.