added aspnetcore package and adapted sample app

Tests/MQTTnet.TestApp.AspNetCore2/wwwroot/app/app.ts

@@ -0,0 +1,16 @@
+import { connect } from "mqtt";
+
+var client = connect('ws://' + location.host + '/mqtt',
+    {
+        clientId: "client1",
+    });
+
+client.on('connect', () => {
+    client.subscribe('presence');
+    client.publish('presence', 'Hello mqtt');
+});
+
+client.on('message', (topic, message) => {
+    // message is Buffer
+    console.log(message.toString());
+})