Merge pull request #1325 from jtattermusch/grpc_nuget

src/csharp/Grpc.Core/.gitignore

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/Grpc.Core.nuspec

src/csharp/Grpc.Core/packages.config

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/Grpc.IntegrationTesting/packages.config

templates/vsprojects/vcxproj_defs.include

vsprojects/grpc_csharp_ext/grpc_csharp_ext.vcxproj

vsprojects/nuget_package/.gitignore

vsprojects/nuget_package/README.md

vsprojects/nuget_package/buildall.bat

vsprojects/nuget_package/grpc.native.csharp_ext.nuspec

vsprojects/nuget_package/grpc.native.csharp_ext.props

vsprojects/nuget_package/grpc.native.csharp_ext.targets