Merge pull request #505 from jtattermusch/csharp_single_dll

src/csharp/GrpcCore/GrpcEnvironment.cs

src/csharp/GrpcCore/Internal/CallSafeHandle.cs

src/csharp/GrpcCore/Internal/ChannelSafeHandle.cs

src/csharp/GrpcCore/Internal/CompletionQueueSafeHandle.cs

src/csharp/GrpcCore/Internal/Event.cs

src/csharp/GrpcCore/Internal/ServerSafeHandle.cs

src/csharp/GrpcCore/Internal/Timespec.cs

src/csharp/GrpcCoreTests/TimespecTest.cs

src/csharp/ext/grpc_csharp_ext.c

...vsprojects/vs2013/grpc_csharp_ext_shared.vcxproj.template

vsprojects/vs2013/grpc_csharp_ext_shared.vcxproj