multiple at_exit hooks are confusing

### Was: How to test (asynchronously) Celluloid::Notifications?

The following test passes when you run `rspec testfile.rb`. But if you run `ruby testfile.rb` you'll get "Celluloid::DeadActorError: notifications fanout actor not running" exception.

How can I keep the test waiting and the actor running? I tried using the `receive` method to keep waiting but without success.

``` ruby
require 'rspec/autorun'
require 'celluloid'

class Foo
  include Celluloid
  include Celluloid::Notifications

  attr_reader :count

  def update_counter(topic, counter)
    @count ||= 0
    @count += counter
  end
end

class Bar
  include Celluloid
  include Celluloid::Notifications

  def increase
    publish('counters', 1)
  end
end

describe Celluloid::Notifications do
  it 'increases the counter' do
    foo = Foo.new
    foo.subscribe('counters', :update_counter)

    bar = Bar.new
    bar.increase

    foo.count.should == 1
  end
end
```