Fission dotnet 2.0 env (#337)

Adding a new dotnet environment, supporting dotnet core 2.0.

Project structure changed. Project.json was abandoned. Project.csproj is now used.

environments/dotnet20/Program.cs

@@ -0,0 +1,21 @@
+using System.IO;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Nancy.Owin;
+
+namespace Fission.DotNetCore
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            var host = new WebHostBuilder()
+               .UseContentRoot(Directory.GetCurrentDirectory())
+               .UseKestrel()
+               .UseUrls("http://*:8888")
+               .Configure(app => app.UseOwin(x => x.UseNancy()))
+               .Build();
+            host.Run();
+        }
+    }
+}