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™.
Why not:
```go
if r.filter != nil {
    return r.filter(err)
}
return true