Merge pull request #2926 from nicolasnoble/cancel_gpr_cancelable

BUILD

Makefile

build.json

gRPC.podspec

include/grpc/support/cancellable_platform.h

include/grpc/support/sync.h

src/core/support/cancellable.c

src/core/support/sync.c

test/core/support/cancellable_test.c

tools/doxygen/Doxyfile.core

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

tools/run_tests/tests.json

vsprojects/Grpc.mak

vsprojects/gpr/gpr.vcxproj

vsprojects/gpr/gpr.vcxproj.filters