Added TLS1.2 support to client and server.

Added MqttClientSslChannel and MqttServerSslChannel which both utilize
an SSL stream.  Added options to both MqttCLientOptions and
MqttServerOptions.  I was unable to update the code in
MQTTnet.UniversalWindows due to my lack of Windows 10.

MQTTnet.Core/Client/MqttClientOptions.cs

@@ -19,5 +19,24 @@ namespace MQTTnet.Core.Client
         public TimeSpan KeepAlivePeriod { get; set; } = TimeSpan.FromSeconds(5);
 
         public TimeSpan DefaultCommunicationTimeout { get; set; } = TimeSpan.FromSeconds(10);
+
+        /// <summary>
+        /// Use SSL to communicate with the MQTT server.
+        /// </summary>
+        /// <remarks>Setting this value to <c>true</c> will also set <see cref="Port"/> to <c>8883</c> if its value was <c>1883</c> (not set).</remarks>
+        public bool UseSSL
+        {
+            get => _useSSL;
+            set
+            {
+                // Automatically set the port to the MQTT SSL port (8883) if it wasn't set already
+                if (value && Port == 1883)
+                    Port = 8883;
+
+                _useSSL = value;
+            }
+        }
+
+        private bool _useSSL;
     }
 }