Fix minimum ping interval handling.

* Change the type of `Worker.pingInt` to `time.Duration`.
* Fix the ping rate detection logic.
* Ensure that `Worker.lastPing` is updated on each `Worker.Ping()` call.
Syncing up with `dev`.
Syncing up with `dev`.
Merge pull request #55 from mozilla-services/bug/ping-interval

Fix minimum ping interval handling