Add a Set() operation to the front API.

As an alternative to #23.

This keeps the Add() call in the frontend API (the thing that writes to etcd), and adds a Set() call as an alternative.

In the backend (the thing that watches for etcd changes), I've changed it to do operate in set() mode only. If I change a key behind strowger's back in etcd directly, there is no reason it shouldn't simply accept that change.