add support for CoreCLR

src/csharp/.gitignore

src/csharp/Grpc.Auth/Grpc.Auth.xproj

src/csharp/Grpc.Auth/project.json

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.xproj

src/csharp/Grpc.Core.Tests/NUnitVersionTest.cs

src/csharp/Grpc.Core.Tests/SanityTest.cs

src/csharp/Grpc.Core.Tests/project.json

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

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

src/csharp/Grpc.Core/Grpc.Core.xproj

src/csharp/Grpc.Core/Internal/DefaultSslRootsOverride.cs

src/csharp/Grpc.Core/Internal/NativeExtension.cs

src/csharp/Grpc.Core/Internal/PlatformApis.cs

src/csharp/Grpc.Core/Internal/SafeHandleZeroIsInvalid.cs

src/csharp/Grpc.Core/project.json

src/csharp/Grpc.Dnx.sln

...p/Grpc.Examples.MathClient/Grpc.Examples.MathClient.xproj

src/csharp/Grpc.Examples.MathClient/project.json

...p/Grpc.Examples.MathServer/Grpc.Examples.MathServer.xproj

src/csharp/Grpc.Examples.MathServer/project.json