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
```
If we're processing a lot of messaging, we might want to bulk the delete operations: http://dev.iron.io/mq/reference/api/#delete_multiple_messages_from_a_queue