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™.
While exponential backoff in many cases is desired, this should probably be a configurable value for when its not desired. Or if someone wants a backoff slightly less than exponential.