Allow a catch-all listener.

This will automatically emit all events to any listener on the `*` event.

Usage:

``` php
<?php

$emitter->addListener('*', function ($event) {
    Logger::debug('Event Triggered: '.$event->getName());
});
```
Added a spec for wildcard listeners.
Merge pull request #4 from dhrrgn/patch-1

Allow a catch-all listener.
Merge branch 'master' of github.com:thephpleague/event
Refactored listener invocation.
Added wildcard listeners to the changelog