Make auth interfaces more flexible

Allow AuthenticationNeeded/GrantNeeded interfaces to decide whether to handle the request, or let the default error response be written by the oauth server
That wins you a [merge]


> On Nov 21, 2014, at 9:20 AM, Jordan Liggitt <notifications@github.com> wrote:
> 
> In pkg/auth/oauth/registry/registry_test.go:
> 
> > @@ -179,15 +195,18 @@ func TestRegistryAndServer(t *testing.T) {
> >  				handlers.NewAuthorizeAuthenticator(
> >  					h,
> >  					h,
> > +					h,
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> 
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