structured message for selector; matching functionality

In response to https://github.com/GoogleCloudPlatform/kubernetes/pull/360. This is an incremental change. To more fully realize the goals outlined therein, a string form would need to be added, deprecated selector code in this commit's files would need to be removed and the structured message would need to be expanded.
It may make sense to store strValues as a util.StringSet, then you don't need this function.