Enable watch on node changes.

Add node watch.  I haven't updated scheduler or node controller to use the watch, since there we can't support filtering based on field before NodeStatus is in place.

Our watch code is not tested (at least the filter function).  I added a rest-level watch test for the new minion watch code.  If it's ok, then I can update other object test also.
I'm following the pattern of Pod.Watch and Controller.Watch, I think both of them filter at the REST layer.  And that's why I want to add a rest level watch test for them.  Did I understand it wrong?

https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/registry/pod/rest.go#L187
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/registry/controller/rest.go#L148

Service & Endpoint registry passes label and field to storage level, but they requires exact match for name field.