add fetch api
	modified:   consume_test.js
	modified:   lib/Consumer.js
	modified:   lib/FetchRequest.js
	modified:   lib/FetchResponse.js
	modified:   lib/Message.js
	modified:   lib/MessageSet.js
	modified:   lib/ProduceRequest.js
	modified:   lib/Request.js
	modified:   lib/Response.js