Add support for winsdk8 (vs 11.0 / vs2012)

Some file paths were changed with visual studio 2012 (version 11.0),
and the new winsdk8:

- Traditional windows API are found under a "UM" (user-mode) directory.
- WinRT (new APIs) are found under a "WINRT" directory
- Others in "SHARED" directory

We import the UM headers for now. WinRT support should come probably with
its own toolset variant.