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.
Why not follow the pattern of other watches and pass label and fields down into the registry?  The reason we do that is so that different underlying data store implementations can do watch efficiently.  If you implement filter at the REST layer, there's no way to have the registry perform optimal field or watch filtering (for instance, if we have a DB backing the store that has an index on a particular field, we should use the field selector to qualify the query, not pass a func).