Make PusherChannel::defaultName public static

[andregce]

I propose we make the defaultName function public static. That way we can use it to generate the beams token.

I want to use it like in the example below (I would place it inside my UserController).

    /**
     * Generate a Pusher Beams auth token to allow a user to associate their device with their user id.
     * The token is valid for 24 hours.
     *
     * @throws Exception
     *
     * @return array
     */
    public function generateBeamsToken()
    {
        $beamsClient = new PushNotifications([
            'instanceId' => Config::get('services.pusher.beams_instance_id'),
            'secretKey' => Config::get('services.pusher.beams_secret_key'),
        ]);

        return $beamsClient->generateToken(PusherChannel::defaultName(Auth::user()));
    }

Would that be ok? Can I submit the PR?

Or maybe there is another way of accomplishing this?

[atymic]

Closed in #70