#144 wait_on_handle -async crashes on invalid handles

[Ashok P. Nadkarni]

If wait_on_handle -async is called with an invalid handle, program crashes. Moreover, note that MSDN says behaviour is undefined if a previous handle is closed while the wait is active (RegisterWaitForObject).

Protect against this by either internally maintaining handle list (currently we rely on Windows API validating handles) or by checking validity through GetHandleInfo before calling Register*