[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'd *really* prefer to see these ifdef's go away. Would it be possible that on WinRT you create emulations of CreateEventX using CreateEventExX? Put them into that platform abstraction header. That removes a lot of the source file churn.