Merge github.com:google/grpc into buffer

Makefile

build.json

include/grpc++/credentials.h

include/grpc/grpc_security.h

include/grpc/support/port_platform.h

src/core/channel/client_channel.c

src/core/iomgr/pollset_kick.c

src/core/security/credentials.c

src/core/security/security_context.c

src/core/statistics/census_rpc_stats.c

src/core/statistics/census_tracing.c

src/core/statistics/census_tracing.h

src/core/support/env.h

src/core/support/env_linux.c

src/core/support/env_posix.c

src/core/support/env_win32.c

src/core/support/file.c

src/core/support/file.h

src/core/support/file_posix.c

src/core/support/file_win32.c