build: support go install github.com/coreos/etcd/cmd/etcd

Could build via github.com/coreos/etcd/cmd but that would generate a binary
named "cmd", which is not ideal.

client/integration/doc.go

@@ -0,0 +1,17 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package integration implements tests built upon embedded etcd, focusing on
+// the correctness of the etcd v2 client.
+package integration