extend selector with EXISTS, new tests/some old tests use table-driven style

Based on discussion in https://github.com/GoogleCloudPlatform/kubernetes/pull/715. Some of the old tests were not converted to table-driven format since the code on which they are based will probably be removed in the near term. Future work includes: changing uses of old code in other parts of the code base to use newer version of selector, removing old selector code, removing tests of old selector code, and adding number ranges to selector values.
#1303 is about creating a type for label selectors. I was suggesting a name for single value matching. We only supported conjunctions of key=value in the API -- i.e., exact matches. Perhaps we should just create the type for the new set-based matching, though, and use that in v1beta3.