Added grpc++.h with mainpage docs.

BUILD

Makefile

build.json

examples/cpp/helloworld/greeter_async_client.cc

include/grpc++/channel.h

include/grpc++/completion_queue.h

include/grpc++/grpc++.h

include/grpc/grpc.h

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/run_tests/sources_and_headers.json

vsprojects/grpc++/grpc++.vcxproj

vsprojects/grpc++/grpc++.vcxproj.filters

vsprojects/grpc++_unsecure/grpc++_unsecure.vcxproj

vsprojects/grpc++_unsecure/grpc++_unsecure.vcxproj.filters