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.

Frameworks/MQTTnet.NetCoreApp/MqttClientFactory.cs

@@ -1,5 +1,6 @@
 using System;
 using MQTTnet.Core.Adapter;
+using MQTTnet.Core.Channel;
 using MQTTnet.Core.Client;
 using MQTTnet.Core.Serializer;
 
@@ -11,7 +12,11 @@ namespace MQTTnet
         {
             if (options == null) throw new ArgumentNullException(nameof(options));
 
-            return new MqttClient(options, new MqttChannelCommunicationAdapter(new MqttTcpChannel(), new DefaultMqttV311PacketSerializer()));
+            return new MqttClient(options,
+                // The cast to IMqttCommunicationChannel is required... Roslyn is too stupid to
+                // figure out how to cast back to the base type
+                new MqttChannelCommunicationAdapter(options.UseSSL ? new MqttClientSslChannel() : (IMqttCommunicationChannel) new MqttTcpChannel(),
+                    new DefaultMqttV311PacketSerializer()));
         }
     }
 }