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/helloworld.cs

@@ -0,0 +1,9 @@
+using Fission.DotNetCore.Api;
+
+public class FissionFunction
+{
+    public string Execute(FissionContext context)
+    {
+        return "Hello World!";
+    }
+}