Add skeletal client and api.AddKnownTypes method

Concrete example:

An APIGroup is a generic way to handle a list of resources: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/apiserver/apiserver.go#L56
A Kubernetes master defines the set of things it supports: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/master/master.go#L116

An apiserver knows what group of supported things it wants to setup: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/cmd/apiserver/apiserver.go#L111