add etcd backend

At the bottom of the [Standard Mode readme section](https://github.com/progrium/ambassadord#standard-mode-aka-boring-but-useful-mode), there's an example using an etcd backend:

```
$ docker run -d progrium/ambassadord etcd://127.0.0.1:4001/path/to/backend/nodes
```

I tried to use it, but quickly realized it wasn't implemented yet. This should take care of that :)