RFC: proxy mode

This only implements the proxy-only mode, it does not support falling back from failling to join the active raft cluster.
@bcwaldon reference http://golang.org//pkg/net/http/httputil/; http://golang.org/pkg/net/http/httptest/

we can have a better pkg name. the pkg name we are using is just `etcdserver`. 
And importing `etcdserver` can avoid conflicts. (some people might want to embedded etcd)