Register event listeners in service definition (not part of the boot)

Since #705, listener declarations are part of the "service definition" as the service `dispatcher` is extended instead if being instanced. I removed the listener declarations from the `boot` method to put them in the `register` method.

That might avoid issues like what I described here: https://github.com/fabpot/Silex/pull/705#issuecomment-17969365 if the `kernel` is instanced (with the dispatcher injected) during the boot.