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.
Ok, I'll file a bug and start a fix.  Meanwhile, can you verify if the general approach is correct?  Apart from the filter layer of course.