[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.

I would use ``$data[] = $this->getCallableData($listener);`` to avoid turning the ``$data`` zval into a reference one (which would probably require copying it when returning it at the end of the method as it does not return by reference. See http://jpauli.github.io/2014/06/27/references-mismatch.html for more explanations).

@jpauli your blog post does not explain how PHP behaves for the case of returned values btw. Is it able to figure that the zval is not used by reference anymore (``array_push`` is done) and it could turn it into a normal zval again when returning it, or will it copy it here ? this might require an update of the blog post