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.
Add `EmceeStore.releaseWithout()`.

`releaseWithout()` ensures that bad connections are not returned to the
memcached connection pool.
Syncing up with `dev`.
Syncing up with `dev`.
Syncing up with `dev`.
Syncing up with `dev`.
Merge pull request #55 from mozilla-services/bug/ping-interval

Fix minimum ping interval handling
Merge pull request #54 from mozilla-services/feature/conditional-releases

Conditionally release memcached clients
Merge branch 'dev' of github.com:mozilla-services/pushgo into feature/add_gomemcache

Conflicts:
	src/github.com/mozilla-services/pushgo/simplepush/emcee_store.go