Add support for browser.on(:response).

* This adds an additional callback for the `Network.responseReceived`
  event when a response is received. It will look up the
  `Ferrum::Network::Exchange` object with the matching `requestId` and passes
  it back to the block, once a response has been received for a request.

lib/ferrum/page.rb

@@ -204,6 +204,14 @@ def on(name, &block)
           exchange.intercepted_request = request
           block.call(request, index, total)
         end
+      when :response
+        @client.on('Network.responseReceived') do |params, index, total|
+          exchange = select(params["requestId"]).last
+
+          if exchange
+            block.call(exchange, index, total)
+          end
+        end
       when :auth
         @client.on("Fetch.authRequired") do |params, index, total|
           request = Network::AuthRequest.new(self, params)