[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.
This isn't how we support WinXP in Boost.Thread. Have a look at include/boost/thread/win32/thread_primitives.hpp where you'll see we use GetProcAddress to query KERNEL32.dll for whether some API is present.