Support syncing and `--no-sync` flag in `etcdctl member` commands

Fix https://github.com/coreos/etcd/issues/1558
I thought I would need to do it for the Sync call, but it turns out I could use the /v2/members endpoint rather than reading X-Cluster-Peers header. I left it in, however, since we will need the Location header to follow redirects. That's coming in a future PR.



> On Oct 31, 2014, at 12:50 PM, Jonathan Boulle <notifications@github.com> wrote:
> 
> In client/http.go:
> 
> > @@ -43,7 +43,7 @@ type httpAction interface {
> >  }
> >  
> >  type httpActionDo interface {
> > -	do(context.Context, httpAction) (int, []byte, error)
> > +	do(context.Context, httpAction) (*http.Response, []byte, error)
> why this change? can't see where resp.^StatusCode is being used. unless I'm just missing it with these old eyes.
> 
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