Add the ability to set the workqueue size

Part 1 of 2 to help address https://github.com/adhearsion/blather/issues/139

* Allow users to set the size of the GirlFriday::WorkQueue by setting
  Blather::Client.queue_size.
* The queue size is copied from the class at instance initialization time,
  so it is possible to make clients with different queue sizes if necessary.
* If the queue size is 0, then #receive_data will process the incoming stanza
  immediately.

Note: as the initializer has no arguments, we felt a class method was the best option here. I figure the most likely case is that a given application will want the same queue size for all of its clients, but if there's a better way to specify that, please let me know.
Maybe this conditional could be removed by defaulting to some dummy queue which just works immediately when the queue size is 0.