Add graceful shutdown. Closes #139.

* Add `IsEtcdKeyNotExist()`.
* `app.Stop()` => `app.Close()`.
* Move `{Set}Locator()` to `Application`.
* Add a `close_delay` option for `Etcd{Locator, Balancer}`. This option
  should be set to 1-2 times the refresh interval.
Use a stronger PRNG seed.