Merge remote-tracking branch 'google/release-0_13' into upmerge-from-0.13

Conflicts:
	Makefile
	build.yaml
	package.json
	setup.py
	src/core/surface/version.c
	src/csharp/Grpc.Core/VersionInfo.cs
	src/csharp/build_packages.bat
	src/python/grpcio/grpc_version.py
	src/ruby/lib/grpc/version.rb
	tools/doxygen/Doxyfile.c++
	tools/doxygen/Doxyfile.c++.internal
	tools/doxygen/Doxyfile.core
	tools/doxygen/Doxyfile.core.internal