Added support for explicit :ack and :reject (set "acknowledge true" in your config file)

The env["rabbit.message"] will contain an object that the worker can use
to ack or recect the message at any point during processing:

  env["rabbit.message"].ack

or

  env["rabbit.message"].reject

If the worker doesn't do anything, the message will automatically get :ack'ed at the end of the
rack handler (if success) or automatically :reject'ed (if failure/exception)