util: Porting NotificationSubscriber, FaceMonitor, and NotificationStream from NFD

New generic classes:
- `util::NotificationSubscriber` (`util/notification-subscriber.hpp`)
- `util::NotificationStream` (`util/notification-stream.hpp`)

NFD Face monitoring class:
- `nfd::FaceMonitor` (`management/nfd-face-monitor.hpp`)

Change-Id: I2ab0a2cd9d7e3ac07036f290f0b4de5eb16e6e38
diff --git a/src/management/nfd-face-monitor.hpp b/src/management/nfd-face-monitor.hpp
new file mode 100644
index 0000000..07af961
--- /dev/null
+++ b/src/management/nfd-face-monitor.hpp
@@ -0,0 +1,72 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2013-2014 Regents of the University of California.
+ *
+ * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
+ *
+ * ndn-cxx library is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU Lesser General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later version.
+ *
+ * ndn-cxx library is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.
+ *
+ * You should have received copies of the GNU General Public License and GNU Lesser
+ * General Public License along with ndn-cxx, e.g., in COPYING.md file.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ *
+ * See AUTHORS.md for complete list of ndn-cxx authors and contributors.
+ */
+
+/**
+ * Original copyright notice from NFD:
+ *
+ * Copyright (c) 2014,  Regents of the University of California,
+ *                      Arizona Board of Regents,
+ *                      Colorado State University,
+ *                      University Pierre & Marie Curie, Sorbonne University,
+ *                      Washington University in St. Louis,
+ *                      Beijing Institute of Technology,
+ *                      The University of Memphis
+ *
+ * This file is part of NFD (Named Data Networking Forwarding Daemon).
+ * See AUTHORS.md for complete list of NFD authors and contributors.
+ *
+ * NFD is free software: you can redistribute it and/or modify it under the terms
+ * of the GNU General Public License as published by the Free Software Foundation,
+ * either version 3 of the License, or (at your option) any later version.
+ *
+ * NFD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+ * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+ * PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * NFD, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef NDN_MANAGEMENT_NFD_FACE_MONITOR_HPP
+#define NDN_MANAGEMENT_NFD_MONITOR_HPP
+
+#include "../util/notification-subscriber.hpp"
+#include "nfd-face-event-notification.hpp"
+
+namespace ndn {
+namespace nfd {
+
+/** \brief A subscriber for Face status change notification stream
+ *  \sa http://redmine.named-data.net/projects/nfd/wiki/FaceMgmt#Face-Status-Change-Notification
+ */
+class FaceMonitor : public util::NotificationSubscriber<FaceEventNotification>
+{
+public:
+  FaceMonitor(Face& face)
+    : NotificationSubscriber<nfd::FaceEventNotification>(face, "ndn:/localhost/nfd/faces/events")
+  {
+  }
+};
+
+} // namespace nfd
+} // namespace ndn
+
+#endif // NDN_MANAGEMENT_NFD_FACE_MONITOR_HPP