Move labels and tests to new package

I also implemented label querying on services and replication controllers. This could conceivably cause some confusion with services, since their "labels" attribute is actually itself used as a query for pods that the service applies to, but I think it'd be even more confusing for it to have both Labels and PodLabels (or some such) attributes, and there doesn't seem to be a compelling need at the moment.
Conveniently