[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.
Yes the WRL threading done here could be used on Windows 8 desktop as well, however there still would have to be different binaries produced for Windows store and phone. Reasons being different libs, like the CRT, are linked against and the /APPCONTAINER requirement. Also environment variables aren't available in the Windows Runtime.

The best that I can think of is using a macro that can be used to turn on/off to utilize WRL instead of the Win32 APIs. This would always be on for Windows store and phone, and optionally could be turned on for desktop (only will work on Windows 8.0 and later).