Merged changes from master

.travis.yml

BUILD

CONTRIBUTING.md

Makefile

build.json

examples/pubsub/main.cc

examples/pubsub/publisher_test.cc

examples/pubsub/subscriber_test.cc

gRPC.podspec

include/grpc++/client_context.h

include/grpc++/completion_queue.h

include/grpc++/credentials.h

include/grpc++/impl/grpc_library.h

include/grpc++/server.h

include/grpc++/server_context.h

include/grpc++/stream.h

include/grpc++/time.h

include/grpc/grpc.h

include/grpc/support/tls.h

src/core/channel/call_op_string.c