add extension_list example (#281)

- Created an example of extension policy
- Added C# binding support of .NET framework 4.0 and created a Nuget
  spec for it.
- Added a pytest in python bindings to test the extension policy and the
  python binding
- Restructured the example and Csharp binding directories due to above
  changes.
- Added copyrights.
- Added a Windows workflow for .NET 4.0 build and test.

bindings/csharp/net40/regorus-test.csproj

@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk" InitialTargets="BuildRegorusFFI"> 
+
+  <Target Name="BuildRegorusFFI">
+    <Exec Command="cargo build -r --manifest-path ../../ffi/Cargo.toml" />
+    <Copy SourceFiles="../../ffi/RegorusFFI.g.cs" DestinationFolder="." />
+    <ItemGroup>
+        <RegorusDylib Include="..\..\..\target\release\*regorus_ffi*" />
+    </ItemGroup>    
+    <Copy SourceFiles="@(RegorusDylib)" DestinationFolder="." />
+  </Target>
+
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net40</TargetFramework>
+    <RootNamespace>regorus_test</RootNamespace>
+    <StartupObject>regoregorus_test.Program</StartupObject>
+    <ImplicitUsings>enable</ImplicitUsings> 
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+
+
+</Project>