Added UDP multicast for plugin discovery (#153)

* Added Timer

* Added UDPSocket::GetAddr

* Added MessageHandlers::SendBeacon()

* PoC of starting a timer on PluginEnable

* C++11

* Added Timer to xcode project

* C++11 in cmake

* added Timer to cmake

* use function pointer in Timer

* moved Timer to namespace

+ wait for thread to join when stopping the timer

* Windows: changed uint to unisgned short

* Windows: Added Timer.h/cpp to project

* GetAddr static

* Send xplane and plugin version with BECN

* SendBeacon with params

* fixed file copyrights

* Include functional

to fix Linux compile error

* review fixes

* Send plugin receive port in BECN

* review fixes

xpcPlugin/Timer.cpp

@@ -0,0 +1,27 @@
+// Copyright (c) 2013-2018 United States Government as represented by the Administrator of the
+// National Aeronautics and Space Administration. All Rights Reserved.
+
+#include "Timer.h"
+using namespace std;
+
+namespace XPC {
+    void Timer::start(const chrono::milliseconds interval, const Callback &callback) {
+        {
+            running.test_and_set();
+            th = thread([=]()
+                        {
+                            while (running.test_and_set()) {
+                                this_thread::sleep_for(interval);
+                                callback();
+                            }
+                        });
+        }
+    }
+    
+    void Timer::stop() {
+        running.clear();
+        if(th.joinable()) {
+            th.join();
+        }
+    }
+}