[winrt support] Enabling Boost.Thread to be used in the Windows Runtime.

This involves basically 3 changes:
1. Using __declspec(thread) instead of the Tls APIs.
2. Using Windows::System::Threading since Win32 Threading APIs aren't allowed.
3. Updating or replacing some banned APIs like WaitForSingleObject with WaitForSingleObjectEx.
I am right in thinking that the WRL works in normal desktop Win8 too? If so, can you replace the ifdefs for this stuff with an environment variable based runtime enabler? The reason why is because I'd like the WRL functionality being regularly unit tested, and my Windows CI here is normal Win8.1 desktop, and we already dump out enough alternative binaries for Boost.Thread on Windows to the point it already confuses end users.