Add option to enable a simple CORS implementation for the api server

Related to https://github.com/GoogleCloudPlatform/kubernetes/issues/1128
I don't see any advantage in enforcing CORS on the server either. You basically reimplementing what's already in any browser that supports CORS. Those that don't support simply won't allow cross-origin requests. It could be as simple as a couple apiserver daemon cmd line opts, no handler logic. The handler would simply relay what was specified on the cmd line in approriate headers.