Add db connection setting

Hi, 

First thanks for the package. 

I'm using your package for an app that uses multiple databases and was not able to find a way in the package to set which db the package will use so I added an option for it and updated some of the codes.

I added db_connection on config/oauth.php and updated Repositories to check if one is set and if it is use it and if not use the default db set on config/database.php

in migrating the package --database will be added on the artisan command like so
php artisan migrate --package="lucadegasperi/oauth2-server-laravel" --database=otherdb

Regards,
Airon