Treating the rake task as the public interface here, but this would
break anyone using the stuff in lib/monitoring

* Created a real data type for the results of checks
* Removed the "per queue" notifier, which was tightly coupled to its
  checker.  Now librato notifier can handle all of them
* This should make it easier to add more advanced checks/notifications.

To get confidence this won't break anything, I basically made the
internal changes and kept running the monitoring integration test.  Once
I'd made all the changes without that breaking, I then updated the unit
tests.

(The change in the monitoring integration test was just to add failure
messages to the asserts. No actual changes were made)