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/tests/unit-tests/util/simple-notification.hpp b/tests/unit-tests/util/simple-notification.hpp
new file mode 100644
index 0000000..2def4bb
--- /dev/null
+++ b/tests/unit-tests/util/simple-notification.hpp
@@ -0,0 +1,123 @@
+/* -*- 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_UNIT_TESTS_UTIL_SIMPLE_NOTIFICATION_HPP
+#define NDN_UNIT_TESTS_UTIL_SIMPLE_NOTIFICATION_HPP
+
+#include "common.hpp"
+
+#include "encoding/encoding-buffer.hpp"
+#include "security/key-chain.hpp"
+
+namespace ndn {
+namespace tests {
+
+class SimpleNotification
+{
+public:
+  SimpleNotification()
+  {
+  }
+
+  explicit
+  SimpleNotification(const Block& block)
+  {
+    wireDecode(block);
+  }
+
+  SimpleNotification(const std::string& message)
+    : m_message(message)
+  {
+  }
+
+  ~SimpleNotification()
+  {
+  }
+
+  Block
+  wireEncode() const
+  {
+    ndn::EncodingBuffer buffer;
+    prependByteArrayBlock(buffer,
+                          0x8888,
+                          reinterpret_cast<const uint8_t*>(m_message.c_str()),
+                          m_message.size());
+    return buffer.block();
+  }
+
+  void
+  wireDecode(const Block& block)
+  {
+    m_message.assign(reinterpret_cast<const char*>(block.value()),
+                     block.value_size());
+
+    // error for testing
+    if (!m_message.empty() && m_message[0] == '\x07')
+      throw tlv::Error("0x07 error");
+  }
+
+public:
+  const std::string&
+  getMessage() const
+  {
+    return m_message;
+  }
+
+  void
+  setMessage(const std::string& message)
+  {
+    m_message = message;
+  }
+
+private:
+  std::string m_message;
+};
+
+} // namespace tests
+} // namespace ndn
+
+#endif // NDN_UNIT_TESTS_UTIL_CORE_SIMPLE_NOTIFICATION_HPP