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.

examples/dotnet20/echo.cs

@@ -0,0 +1,10 @@
+using System;
+using Fission.DotNetCore.Api;
+
+public class FissionFunction 
+{
+    public string Execute(FissionContext context){
+        context.Logger.WriteInfo("executing.. {0}", context.Arguments["text"]);
+        return (string)context.Arguments["text"];
+    }
+}