Implement threads.condvars for Windows in an XP-compatible way

[jckarter]

I implemented threads.condvars for Windows using the ConditionVariable APIs introduced in Vista. Windows XP doesn't have native condition variables, so a backward-compatible implementation should be available too. See http://www.cse.wustl.edu/~schmidt/win32-cv-1.html for a discussion of implementing condition variables using Win32 concurrency primitives pre-Vista.