Fixed up the Response Headers (X-Etcd-Index, etc.)

Thanks for showing me where to start! That commit didn't work, though, so I did my best to fix it up. What do you think?

Commit Message:
Fixing the three Headers now included on an EtcdResponse, including the tests. EtcResponseHelpers has been removed as it isn't used and throws exceptions on use. Setting the address in EtcdBase back to localhost (127.0.0.1). It's worth noting that EtcdIndex is expected but both RaftIndex and RaftTerm are nullable; I noticed that, in a few tests (Compare and Delete being one), these two headers were not returned by etcd even when X-Etcd-Index was.