add string form, 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.
Is it not valid for a label to have an empty value?  That would fail this test but the label would actually exist.

It would seem clearer to add a Has() method to the Labels interface, and test that here.