[FrameworkBundle] [Command] Event Dispatcher Debug - Display registered listeners

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

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[Update] The PR has been updated in order to comply with @stof comments.

Current status : 
- [x] New event dispatcher Descriptor
- [x] Manage all callables
- [x] Unit tests
- [x] Text description
- [x] XML description
- [x] Json description
- [x] Markdown description

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Hi. In some big applications with lots of events, it's often hard to debug which classes listen to which events, and what is the order of theses listeners. This PR allows to run

- *event-dispatcher:debug* which displays all configured listeners + the events they listen to

![capture d cran de 2014-03-07 20 13 56](https://f.cloud.github.com/assets/1172099/2361104/40a86a62-a62d-11e3-9ccd-360a8d75b2a4.png)
 
- *event-dispatcher:debug* **event** which displays configured listeners for this specific event (order by priority desc)

![capture d cran de 2014-03-07 20 14 31](https://f.cloud.github.com/assets/1172099/2361100/31e0d12c-a62d-11e3-963b-87623d05642c.png)

The output is similar to *container:debug* command and is available in all supported formats (txt, xml, json and markdown).

I found another PR with same goal (#8234), but the approach looks too complicated to me plus I think we should fetch the listeners directly with the event_dispatcher.

In fact this is the same logic applied for the router and container descriptions (https://github.com/matthieuauger/symfony/blob/feature-framework-bundle-event-dispatcher-command/src/Symfony/Bundle/FrameworkBundle/Console/Descriptor/XmlDescriptor.php#L144). We should keep consistency between theses functions, don't you think ?

Another thing is that you can describe a callable alone (https://github.com/matthieuauger/symfony/blob/feature-framework-bundle-event-dispatcher-command/src/Symfony/Bundle/FrameworkBundle/Console/Descriptor/Descriptor.php#L73). In this case the creation of the DOMDocument here seems pertinent