[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.
Yet more binary variants for Windows! :( We actually have a magic build flag now for building all the variants, we normally only produce a subset. It should be easy enough to fold the RT binaries in those though, assuming Boost.Build merges RT build support sometime soon (probably after the 1.56 release).