[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.
A number of copyright headers seem to have become reordered. Weird. Anyway, apart from that this patch looks good enough for me to try building. Unfortunately for you we have the first beta of Boost v1.56 in the works, and this Saturday will be lost on getting my CI soak testing code for 24 hours and other such fun. If no nasty surprises turn up, I'll get to this the following Saturday, but if I miss that date I'm on a business trip the following weekend so it could potentially be a month out before I get back to this. Is this okay?