add http health checks.

Is there an escape hatch for extensible check types managed from the server or implemented on the host?  I.e. the difference between what kubernetes implements/specs, vs what someone using the kubernetes API might want to implement?  Could someone decide to say, implement healthcheck type="mysql" but the implementation of that might be left to an external consumer (someone who is watching etcd, integrating with the kubelet locally, or calling the pod api)?