Add the ability to set the workqueue size

* Allow users to set the size of the GirlFriday::WorkQueue by setting
  Blather::Client.queue_size.

* The queue size is passed in as a named parameter in the last argument to
  Blather::Client.setup

* If the queue size is 0, then #receive_data will process the incoming stanza
  immediately.
Merge pull request #140 from emschwar/specify_size_of_workqueue

Add the ability to set the workqueue size