Add basic Authorization.

Added basic interface for authorizer implementations.
Added default "authorize everything" and "authorize nothing
implementations.
Added authorization check immediately after authentication check.
Added an integration test of authorization at the HTTP level of
abstraction.

test/integration/client_test.go

@@ -44,6 +44,7 @@ func TestClient(t *testing.T) {
 		EnableLogsSupport: false,
 		EnableUISupport:   false,
 		APIPrefix:         "/api",
+		AuthorizationMode: "AlwaysAllow",
 	})
 
 	s := httptest.NewServer(m.Handler)