started using nuget packages for dependencies

src/csharp/.gitignore

src/csharp/GrpcApi/.gitignore

src/csharp/GrpcApi/GrpcApi.csproj

src/csharp/GrpcApi/packages.config

src/csharp/GrpcApiTests/.gitignore

src/csharp/GrpcApiTests/GrpcApiTests.csproj

src/csharp/GrpcApiTests/packages.config

src/csharp/GrpcCore/.gitignore

src/csharp/GrpcCoreTests/.gitignore

src/csharp/GrpcCoreTests/GrpcCoreTests.csproj

src/csharp/GrpcCoreTests/packages.config

src/csharp/InteropClient/.gitignore

src/csharp/InteropClient/InteropClient.csproj

src/csharp/InteropClient/packages.config

src/csharp/MathClient/.gitignore

src/csharp/lib/Google.ProtocolBuffers.dll