Add nuget targets file for cross plat packaging (#512)

Author: jonko0493

bindings/csharp/Keystone.Net/.nuget/Keystone.Net.targets

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup>
+    <NativeLibs Include="$(MSBuildThisFileDirectory)win-x64\*.dll"
+      Condition="($([MSBuild]::IsOsPlatform('Windows')) And ('$(Platform)' == 'x64' Or '$(Platform)' == 'AnyCPU') And '$(RuntimeIdentifier)' == '') Or '$(RuntimeIdentifier)' == 'win-x64'" />
+    <NativeLibs Include="$(MSBuildThisFileDirectory)win-x86\*.dll"
+      Condition="($([MSBuild]::IsOsPlatform('Windows')) And '$(Platform)' == 'x86' And '$(RuntimeIdentifier)' == '') Or '$(RuntimeIdentifier)' == 'win-x86'" />
+    <NativeLibs Include="$(MSBuildThisFileDirectory)linux-x64\*.so"
+      Condition="($([MSBuild]::IsOsPlatform('Linux')) And '$(RuntimeIdentifier)' == '') Or '$(RuntimeIdentifier)' == 'linux-x64'" />
+    <NativeLibs Include="$(MSBuildThisFileDirectory)osx-x64\*.dylib"
+      Condition="($([MSBuild]::IsOsPlatform('OSX')) And '$(RuntimeIdentifier' == '') Or '$(RuntimeIdentifier)' == 'osx-x64'" />
+    <None Include="@(NativeLibs)">
+      <Link>%(FileName)%(Extension)</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+      <CopyToPublishDirectory>PreserveNewest</CopyToPublishDirectory>
+    </None>
+  </ItemGroup>
+</Project>