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/MqttServerFactory.cs

@@ -1,4 +1,5 @@
 using System;
+using MQTTnet.Core.Adapter;
 using MQTTnet.Core.Server;
 
 namespace MQTTnet
@@ -9,7 +10,8 @@ namespace MQTTnet
         {
             if (options == null) throw new ArgumentNullException(nameof(options));
 
-            return new MqttServer(options, new MqttServerAdapter());
+            // The cast to IMqttServerAdapter is required... stupidly...
+            return new MqttServer(options, options.UseSSL ? (IMqttServerAdapter)new MqttSslServerAdapter() : new MqttServerAdapter());
         }
     }
 }