Add support for yield-generators over RPC

Hi Alexander,

Here are finally my modifications for supporting yields :).
I could not test very well with the SyncRPCService, and I did not touched the tornado implementation. But it should work for gevent. In addition, it should be rather easy to port it to tornado. There is a reusable _yielder() function in RPCClientBase for the client part. For the service part, one has to copy/paste and adapt the logic I implemented in GeventRPCService.


Cheers,
Axel