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).
There's a slight inconsistency here with line 56-58 (uint/int stuff). Can't you just share the code for `{Create,Set,Update}.HTTPRequest`? Only difference is `prevExist` right?