Notify when transitioning to red

Fixes #7.

I still need to implement the HipChat adapter, but the general idea won't change.

This is going to complicate #12 since it adds another request to the data store in `#run`.

:pager: @clifton 
``` rb
      @notifiers = [*notifiers] if notifiers
```

à la [Postel's law](http://en.wikipedia.org/wiki/Postel's_Law). :thought_balloon::question: