Add listener for tx message and headers message.

[Yamaguchi]

This PR allows bitcoinrb to notify that it receives tx message and headers message.

[Yamaguchi]

To use observer, must implement #update(event, data) method.

class Watcher 
  def update(event, data)
    case event
    when :tx
      # TODO :
    when :header
      # TODO :
    end  
  end
end

watcher = Wather.new
spv.add_observer(watcher)