added etcdIndex to response

Hi again,

for watches we need the X-Etcd-Index header of a response, see \[0\] and \[1\].

Greetings, Marc

\[0\] https://github.com/coreos/etcd/blob/9ba35bc95e51c1bb219226a522e2a613bfa1ba34/Documentation/api.md#response-headers
\[1\] https://github.com/coreos/etcd/pull/1082#issuecomment-56444616