Add simple Bearer authenticator filter for Kube

Now with 100% more implementation. Step 1 of implementing #1430.  Sourced from https://github.com/smarterclayton/origin/commit/b3ccda08e51a4b129348244e7b6c03ccc54750f3

@erictune
I mentally warred between brevity and slavish adherence to verbing interfaces.

----- Original Message -----
> > +See the License for the specific language governing permissions and
> > +limitations under the License.
> > +*/
> > +
> > +package authenticator
> > +
> > +import (
> > +	"net/http"
> > +
> > +	"github.com/GoogleCloudPlatform/kubernetes/pkg/auth/api"
> > +)
> > +
> > +// Token checks a string value against a backing authentication store and
> > returns
> > +// information about the current user and true if successful, false if not
> > successful,
> > +// or an error if the token could not be checked.
> > +type Token interface {
> 
> I guess authenticator.Token is like cipher.Block.  You have to reverse the
> package and interface names to get a good name.  I don't like it at all, but
> I guess it is the Go way.  So, I guess leave it.
> 
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> https://github.com/GoogleCloudPlatform/kubernetes/pull/1529/files#r18408791