Enable load-balancer handling, and configuration options.

When running an app behind both CloudFlare and a load balancer (such as AWS ELB) the remote IP address can be misreported. This is particularly the case if you are expecting requests to come both via CloudFlare -> ELB and also directly through ELB.

Added a 'CloudFlareBehindLoadBalancer' directive that suggests that the remote IP should be processed from a load balancer before being checked for the cloudflare header.

Also added DenyAllButLoadBalancer to return 403 unless a request has come via a specified load balancer.

Out of the box, configured defaults such that enabling CloudFlareBehindLoadBalancer handles ELB requests with no further configuration required.