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.