Add `retry.Helper`.
Retry failed test notifications.
Travis: Test on tip-of-tree Go.
`retry.Helper.Filter` => `CanRetry`.

* Document `RetryFunc` and `RetryAttempt`.
Expose `retry.Helper.Backoff`.
`RetryAttempt`: Fix the backoff algorithm.

`RetryAttempt` now returns the expected delay in addition to indicating
whether the operation can be retried.

`RetryFunc` now returns the correct number of attempts if the retry
was cancelled.
Allow `MaxJitter` = 0.
Add retry tests.
Fix the `golang.org/x` repo tag.
Merge pull request #145 from mozilla-services/feature/retries

Extract retry logic into a common helper