kubectl delete command: adding  labelSelector

@lavalamp @smarterclayton basically I've rewritten the PR integrating your comments.

@smarterclayton: I didn't fix shortcut expansion since I have hard time to understand where the shortcut should come from. Maybe we may discuss about it. 
Would also suggest that `RestHelper.Get(namespace, "", labelSelector)` be moved to its own method - `RestHelper.List(namespace, labelSelector)` which is much closer to our client APIs.  Then `ResourceInfo` and `ResourceSelector` can call Get or List as appropriate.