Extract retry logic into a common helper

* Use `retry.Helper` for `EtcdLocator`. Once #144 lands, the `EtcdBalancer` can use this, too.
* Retry failed test notifications. The tests now consistently pass for me on OS X.
* Test on tip-of-tree Go, because we want to live on the edge™.
Sure thing—I borrowed "filter" from [poolee](https://github.com/dannycoates/poolee#retryfilter). How does `CanRetry` sound?