Move Makefile.msvc under lib
src: Move libevent related helper functions to libevent_util
Add C++ library libnghttp2_asio on top of libnghttp2

The libnghttp2_asio library is C++ library built on top of libnghttp2.
Currently, it has server API and easily create HTTP/2 server using
node.js like API calls.  See the example server source code in
examples/asio-sv.cc.  The library depends on Boost::ASIO library.
Update doc
Add libnghttp2_asio.pc to .gitignore
doc: Add doc how to enable multi threading in libnghttp2_asio
src: Silence compiler warning