Merge branch 'master' into untypedAPI

Makefile

include/grpc++/config.h

include/grpc++/credentials.h

include/grpc/grpc_security.h

include/grpc/support/atm_gcc_atomic.h

include/grpc/support/atm_gcc_sync.h

include/grpc/support/atm_win32.h

src/compiler/config.h

src/compiler/cpp_generator.cc

src/compiler/cpp_generator.h

src/compiler/cpp_generator_helpers.h

src/compiler/cpp_plugin.cc

src/compiler/generator_helpers.h

src/compiler/python_generator.cc

src/compiler/python_plugin.cc

src/core/channel/metadata_buffer.c

src/core/iomgr/fd_posix.c

src/core/security/credentials.c

src/core/security/credentials.h

src/core/security/google_default_credentials.c