Add benchmark app.

Tests/MQTTnet.Benchmarks/Program.cs

@@ -0,0 +1,14 @@
+using System;
+using BenchmarkDotNet.Running;
+
+namespace MQTTnet.Benchmarks
+{
+    public static class Program
+    {
+        public static void Main(string[] args)
+        {
+            var summary = BenchmarkRunner.Run<MessageProcessingBenchmark>();
+            Console.ReadLine();
+        }
+    }
+}