Merge pull request #23 from BoostGSoC/WinIOCP_backend_refactor

Win iocp backend refactor

It looks good to me.
Merge pull request #29 from BoostGSoC/WinIOCP_backend_refactor

Win iocp backend refactor redo
Updated docs.
Merge branch 'master' into MSVC_build
Merge branch 'MSVC_build' of github.com:BoostGSoC/boost.afio into MSVC_build