Generators for threading

Hi Alexander,

Here are the changes to bring support for yield-generators to the threading module.
I found out there is a lot of code duplication between gevent and threading now. I did not factorized them out into netcall/base.py. The variations where mainly on types of objects (Queues, Event, Future/AsyncResult) which would have required a level of abstraction just for them. I did not see how to do it cleanly, but if you do, go ahead ;). Also, the sync client would not have benefited from it unless we turn inside out the factorizable parts of gevent and threading (ie. turn them into similar yielders).

Cheers,
Axel