Add more backends, upstream

[bprashanth]

I'm excited about this project. We need a bare metal ingress controller that supports pluggable backends. I'd be more than happy to help get this as a Kubernetes cluster addon for the baremetal use case (check it into contrib? https://github.com/kubernetes/contrib/tree/master/Ingress/controllers, though that's not a requirement).

Additionally, it would be great if you could adapt https://github.com/kubernetes/contrib/tree/master/service-loadbalancer and https://github.com/kubernetes/contrib/tree/master/Ingress/controllers/nginx-third-party as pluggable backends.