Upload to S3 asynchronously with eventmachine and happening library

async_upload_s3.rb

require 'securerandom'
require 'eventmachine'
require 'happening'

EM.run do
  count = 10
  pending = count

  count.times do |index|
    on_error = Proc.new do |http|
      puts "An error occured: #{http.response_header.status}"
      EM.stop
    end

    key = "#{SecureRandom.uuid}.jpg"
    item = Happening::S3::Item.new(ENV['AWS_S3_BUCKET_NAME'], key,
                                   :aws_access_key_id => ENV['AWS_ACCESS_KEY_ID'],
                                   :aws_secret_access_key => ENV['AWS_SECRET_ACCESS_KEY'],
                                   :permissions => 'public-read')
    filepath = '/DEVELOPMENT/ODESK/simple_prints/SP/server/StoryTree/tests/fixtures/image7.jpg'
    item.put(File.read(filepath), :on_error => on_error,
             :headers => { "Content-Type" => "image/jpeg" }) do |response|
      puts "Upload #{index} finish #{item.url}"
      pending -= 1
      EM.stop if pending == 0
    end
  end
end

I am considering to use 'em-synchrony' to avoid callback hell.