Adds 'certificate required' alert

alert.go

@@ -38,6 +38,7 @@ const (
 	alertInappropriateFallback  alert = 86
 	alertUserCanceled           alert = 90
 	alertNoRenegotiation        alert = 100
+	alertCertificateRequired    alert = 116
 	alertNoApplicationProtocol  alert = 120
 	alertSuccess                alert = 255 // dummy value returned by unmarshal functions
 )