Merge branch 'master' of https://github.com/grpc/grpc into zookeeper

BUILD

INSTALL

Makefile

build.json

include/grpc++/auth_context.h

include/grpc/grpc.h

src/core/channel/client_channel.c

src/core/security/google_default_credentials.c

src/core/surface/call.c

src/core/surface/version.c

src/core/tsi/transport_security_interface.h

src/cpp/common/auth_property_iterator.cc

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

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

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

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

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

...p/Grpc.Core.Tests/Internal/MetadataArraySafeHandleTest.cs

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

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