Add DeleteRecords operation to Broker

[ongardie-ebay]

I added the DeleteRecords operation. Hope it's welcome.

One thing I wasn't sure about is how you'd want the request struct to look. The current map in DeleteRecordsRequestTopic doesn't allow much room for expansion without breaking users. Is that OK?

[ongardie-ebay]

Here's some sample code if you're interested in taking this for a spin:

	broker := sarama.NewBroker("localhost:9092")
	config := sarama.NewConfig()
	config.Version = sarama.V0_11_0_0
	err := broker.Open(config)
	if err != nil {
		panic(err)
	}

	request := sarama.DeleteRecordsRequest{
		Topics: map[string]*sarama.DeleteRecordsRequestTopic{
			"bar": &sarama.DeleteRecordsRequestTopic{
				PartitionOffsets: map[int32]int64{
					0: 2,
					1: 2,
				},
			},
			"foo": &sarama.DeleteRecordsRequestTopic{
				PartitionOffsets: map[int32]int64{
					0: 0,
					1: 2,
				},
			},
		},
	}
	response, err := broker.DeleteRecords(&request)
	if err != nil {
		panic(err)
	}

	fmt.Printf("response:\n")
	for topic, detail := range response.Topics {
		fmt.Printf("topic %v:\n", topic)
		for partition, pdetail := range detail.Partitions {
			errStr := "none"
			if pdetail.Err != 0 {
				errStr = fmt.Sprintf("'%s'", pdetail.Err.Error())
			}
			fmt.Printf("  partition %v: lowWatermark=%v, err=%v\n",
				partition, pdetail.LowWatermark, errStr)
		}
	}

[ongardie-ebay]

Rebased onto current master, fixing trivial conflict in requests.go.

[eapache]

Thanks! Sorry it took me a while to get to this.