some nuget packaging adjustments

src/csharp/Grpc.nuspec

@@ -2,7 +2,7 @@
 <package>
   <metadata>
     <id>Grpc</id>
-    <title>gRPC</title>
+    <title>gRPC C#</title>
     <summary>C# implementation of gRPC - an RPC library and framework</summary>
     <description>C# implementation of gRPC - an RPC library and framework. See project site for more info.</description>
     <version>0.5.0</version>

src/csharp/build_packages.bat

-@rem Builds NuGet packages
+@rem Builds gRPC NuGet packages
 
 @rem Adjust the location of nuget.exe
 set NUGET=C:\nuget\nuget.exe
 
+setlocal
+cd ..\..\vsprojects\nuget_package
 @call buildall.bat || goto :error
+endlocal
 
+@call buildall.bat || goto :error
+
+%NUGET% pack ..\..\vsprojects\nuget_package\grpc.native.csharp_ext.nuspec || goto :error
 %NUGET% pack Grpc.Core\Grpc.Core.nuspec || goto :error
 %NUGET% pack Grpc.Auth\Grpc.Auth.nuspec || goto :error
 %NUGET% pack Grpc.nuspec || goto :error

vsprojects/nuget_package/buildall.bat

 @echo off
-setlocal
 
 REM setlocal
 REM call "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat" amd64