[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.
Actually I took a further look and I don't think __declspec(thread) should be used in all cases. According to msdn on pre-Vista machines a protection fault or corruption can occur.

http://msdn.microsoft.com/en-us/library/2s9wt68x.aspx

I'm also not certain if CE supports or not, I can't seem to find confirmation on msdn. I'm going to leave this as is.