Copy store.Response into go-etcd.

@xiangli-cmu @philips The `go-etcd` library currently depends on `etcd` in order to use the `store.Response` type. This pull request simply copies the `Response` over to the `go-etcd` library.

Is there a test harness for running the tests? I think we should move these functional tests to the `etcd-test-runner` repo once it's created and just leave unit tests in the repo itself.