[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.
The (C) Microsoft may cause problems for the steering committee. As you will note throughout all Boost code, we don't copyright non-individuals, what happens is that the individual is working for whatever corporate employer but gets approval to personally take the copyright. Would Microsoft be able to give you personally the copyright? I know Eric Niebler used to work for Microsoft, he may be able to advise on what Microsoft channels to use.