Add kafka client

* Adds kafka client
* Uses the Hermann gem

*Test Script*
```ruby
require 'stapfen'

class MyWorker < Stapfen::Worker
  use_kafka!

  configure do
    {
      :topic => 'test',
      :groupId => 'groupId',
      :zookeepers => 'localhost:2181'
    }
  end

  consume 'topic/test' do |message|
    puts "Recv: #{message.body}"
  end
end

MyWorker.run!
```
I missed this earlier, sorry, but this RDoc is not correct