Adding events to logging. Fixes #494.

Anytime when you log now, an event, `illuminate.log` will be fired.

Additionally, there is a method, `listen`, on the logging class which you can tap into to observe logging.

```php
Log::listen(function($level, $parameters)
{
    //
});
```

Putting in a separate PR for that on `laravel/laravel`. Will update this PR with details.

Signed-off-by: Ben Corlett <bencorlett@me.com>