add RFC952 validation to labels

Add label validation of labels to  pods, services, relicationControllers, and replControllerState

Fixes #1297
@thockin true and I will make the tests more educational about the labels its testing.    Originally, you and Eric Tune added a-zA-Z in https://github.com/GoogleCloudPlatform/kubernetes/issues/1297.

Should I add _ and . to the regex because RFC952 allows them?