Iron pulling subscriber

This pull request adds a pulling ironmq subscriber to the project.

An example of its usage is as follows.

```ruby
subscriber = RailsPipeline::IronmqPullingSubscriber.new "your favorite queue name"
subscriber.start_subscription do |envelope|
    $stdout.puts envelope.inspect 
end
```
In general I do agree with you, but I would prefer to not hold on to message ids in memory. I could see a scenario where a request for message deletion take a bit too long and the messages are added back on the queue