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/Function.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Reflection;
+using Fission.DotNetCore.Api;
+
+namespace Fission.DotNetCore.Compiler
+{
+    class Function
+    {
+        private readonly Assembly _assembly;
+        private readonly Type _type;
+        private readonly MethodInfo _info;
+
+        public Function(Assembly assembly, Type type, MethodInfo info)
+        {
+            if (info == null) throw new ArgumentNullException(nameof(info));
+            if (assembly == null) throw new ArgumentNullException(nameof(assembly));
+            if (type == null) throw new ArgumentNullException(nameof(type));
+            _assembly = assembly;
+            _type = type;
+            _info = info;
+        }
+
+        public object Invoke(FissionContext context)
+        {
+            return _info.Invoke(_assembly.CreateInstance(_type.FullName), new[] { context });
+        }
+    }
+}