client: add response unmarshal benchmark

The benchmark result:

```
BenchmarkSmallResponseUnmarshal	  10000	   164524 ns/op
BenchmarkManySmallResponseUnmarshal	    100	 13916636 ns/op
BenchmarkMediumResponseUnmarshal	   1000	  1974295 ns/op
BenchmarkLargeResponseUnmarshal	     20	 80462001 ns/op
ok		github.com/coreos/etcd/client	7.777s
```

client/keys_bench_test.go

@@ -0,0 +1,73 @@
+package client
+
+import (
+	"encoding/json"
+	"net/http"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func createTestNode(size int) *Node {
+	return &Node{
+		Key:           strings.Repeat("a", 30),
+		Value:         strings.Repeat("a", size),
+		CreatedIndex:  123456,
+		ModifiedIndex: 123456,
+		TTL:           123456789,
+	}
+}
+
+func createTestNodeWithChildren(children, size int) *Node {
+	node := createTestNode(size)
+	for i := 0; i < children; i++ {
+		node.Nodes = append(node.Nodes, createTestNode(size))
+	}
+	return node
+}
+
+func createTestResponse(children, size int) *Response {
+	return &Response{
+		Action:   "aaaaa",
+		Node:     createTestNodeWithChildren(children, size),
+		PrevNode: nil,
+	}
+}
+
+func benchmarkResponseUnmarshalling(b *testing.B, children, size int) {
+	header := http.Header{}
+	header.Add("X-Etcd-Index", "123456")
+	response := createTestResponse(children, size)
+	body, err := json.Marshal(response)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.ResetTimer()
+	newResponse := new(Response)
+	for i := 0; i < b.N; i++ {
+		if newResponse, err = unmarshalSuccessfulKeysResponse(header, body); err != nil {
+			b.Errorf("error unmarshaling response (%v)", err)
+		}
+
+	}
+	if !reflect.DeepEqual(response.Node, newResponse.Node) {
+		b.Errorf("Unexpected difference in a parsed response: \n%+v\n%+v", response, newResponse)
+	}
+}
+
+func BenchmarkSmallResponseUnmarshal(b *testing.B) {
+	benchmarkResponseUnmarshalling(b, 30, 20)
+}
+
+func BenchmarkManySmallResponseUnmarshal(b *testing.B) {
+	benchmarkResponseUnmarshalling(b, 3000, 20)
+}
+
+func BenchmarkMediumResponseUnmarshal(b *testing.B) {
+	benchmarkResponseUnmarshalling(b, 300, 200)
+}
+
+func BenchmarkLargeResponseUnmarshal(b *testing.B) {
+	benchmarkResponseUnmarshalling(b, 3000, 2000)
+}