Replace go-etcd

This replaces the dependencies on github.com/coreos/go-etcd and github.com/coroes/etcd with a new local package surprisingly called "etcd". It is only intended to implement the functionality that fleet needs (for now).
I guess it would be more like: `cas, err := etcd.Set(key, val, ttl)`. You could add more return params for the interesting fields you need.