Various cleanups to improve debug output

The primary feature is outputting the cluster peers to stderr when
--debug is added.

This is to fix #35 

Example output now:

```
./etcdctl --debug --peers 192.168.1.237:4002,192.168.1.233:4003 set foobar 2139081094801
Cluster-Peers: 192.168.1.237:4002 192.168.1.233:4003
Cannot sync with the cluster
```

```
./etcdctl --debug set foobar 2139081094801
Cluster-Peers: http://127.0.0.1:4001 http://127.0.0.1:4003 http://127.0.0.1:4002
Curl-Example: curl -X PUT http://127.0.0.1:4001/v2/keys/foobar -d value=2139081094801
2139081094801
```