[server][plugin][gate][JSON] Add an AMQP and JSON based arm plugin

It just outputs received message and its content type for now. You can
push a message by the following Ruby script:

    #!/usr/bin/env ruby

    require "bunny"
    require "json"

    server_id = 4 # CHANGE ME

    connection = Bunny.new
    connection.start

    channel = connection.create_channel
    queue = channel.queue("hatohol.gate.#{server_id}")
    queue.publish(JSON.generate({"XXX" => "YYY"}),
                  :content_type => "application/json")
    connection.close

It requires AMQP 0-9-1 supported AMQP broker such as RabbitMQ. And it
also requires librabbitmq library that is a C implementation of AMQP.

TODO:

  * Add API to register hosts
  * Add API to register events
connect() returns false, for example, when an AMQP broker is unexpectedly out of service, doesn't it.
In that case, this thread exits and cannot work after the AMQP broker recovers.
So we should retry to connect taking into account the exit request.