add RFC952 validation to labels

Add label validation of labels to  pods, services, relicationControllers, and replControllerState

Fixes #1297
Yes, I made a mistake by typing underscore instead of minus sign (-).   The current regex already supports minus sign.  I added support for period (.), unless you want me to remove it.

I will be added validation for annotations this morning, as you suggested above, and I will update labels.md to reflect these changes (current doc reflects that syntax is under discussion).

I am also going to change (again) the labels used in the tests to make the RFC952 rules explicitly clear.