I'd like some feedback on this patch.  Is the API acceptable?

Would it be better to throw an exception in wait() instead of returning
None?  

What should communicate() return if it times out?  I can't decide if it
should try to return partial output, return None, or raise an exception.
 If it doesn't return partial output, that output is not recoverable. 
Maybe it should go into the exception object.  On the other hand, the
way that communicate() is implemented with threads on Windows makes it
hard to "interrupt" the file descriptor read and return partial output.
 For that matter, I'm not even sure how to stop those extra threads, as
you can see in the patch.  If anyone can think of a way to avoid using
threads entirely, that would be even better.

What should call() and check_call() return when they timeout?  If they
return None, which is the current returncode attribute, there is no way
of interacting with the process.  They could throw an exception with a
reference to the Popen object.