Allow for IoC injection in addition to Facade

[mattzuba]

Thanks for putting this package together, love it! I prefer to avoid facades if I can and I use dependency injection in most of my code. It'd be great if you could modify the Service Provider to support this. It's a pretty simple change.

$this->app->bind(['pushbullet' => 'Lahaxearnaud\LaravelPushbullet\LaravelPushbullet'], function () {