Added option to explicitily ask for async.callback deletion.

See also: alebsack/rack-fiber_pool#2

In mperham/rack-fiber_pool#24 you changed the `async.callback` to `async.orig_callback`.

As you can see here https://github.com/faye/faye-websocket-ruby/blob/master/lib/faye/websocket.rb#L72, that made this gem not working.
Why did you make that change? 

In my fork I put it under a configuration variable so is not universal by default.

Hope this helps.
Thanks and regards.
  Shogun