Displays friendly message if the event does not have any registered listeners

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

Currently if you run debug:event-dispatcher on an event without registered listeners, the output looks like this

![capture d ecran de 2014-10-13 00 43 26](https://cloud.githubusercontent.com/assets/1172099/4607814/486213e4-5261-11e4-93b3-6ef009c19a05.png)

This PR just add a friendly message like debug:container or debug:router :

![capture d ecran de 2014-10-13 00 44 38](https://cloud.githubusercontent.com/assets/1172099/4607819/7a28a01e-5261-11e4-9b07-d0ca36725da2.png)