Pass action to subscribe() listeners

It'd be nice to be able to learn what actions trigger a subscribe() event:

``` js
store.subscribe(action => {
  if (action.type === '...') {
    /* do something */
  }
})
```

This way you can create listener services that wait for a certain `action` to do something. Example use case is to send Google Analytics data when `ROUTE_CHANGE` occurs (assuming your router sends this out).
