Add node status to API object.

@smarterclayton @lavalamp I'm not sure if this is enough to get it work.  Beat me up.  

 @smarterclayton Should I also take care of v1beta3?

Here "Vanished" is defined as not reachable from the cluster, i.e. connection refused from the node's kubelet port.  Ocasional network failure or machine restarts can make a node unreachable, so we'll need a policy surround the definition, like N out of M tries, at most no connection in X min, etc.  I plan to enforce this on node-controller.