callback argument for setup_nonblocking_producer

Tornado expects callback arguemnts to be namend "callback"
for example to allow the following code to work:

  yield gen.Task(tcelery.setup_nonblocking_producer)

Therefor add "callback" to setup_nonblocking_producer but
keep "on_ready" around for backward compatibility
Merge pull request #48 from FlorianLudwig/master

callback argument for setup_nonblocking_producer