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/requestheaders.cs

@@ -0,0 +1,17 @@
+using System;
+using Fission.DotNetCore.Api;
+
+public class FissionFunction
+{
+    public string Execute(FissionContext context){
+        var buffer = new System.Text.StringBuilder();
+        foreach(var header in context.Request.Headers){
+                buffer.AppendLine(header.Key);
+                foreach(var item in header.Value){
+                        buffer.AppendLine($"\t{item}");
+                }
+        }
+        buffer.AppendLine($"Url: {context.Request.Url}, method: {context.Request.Method}");
+        return buffer.ToString();
+    }
+}