Added support for servers who wish to use a MySQL database to store all the vouchers.

When you update, a new section for the database details will be generated in the config.yml, the database is disabled by default.

If you enable the database setting after the server loads the plugin, you will need to reload the SERVER, not the plugin! Performing a reload/startup with the database option enabled will generate the required tables.

Vouchers are reloaded from the database every time you edit, create, or remove one. Vouchers don't refresh on multiple servers automatically. If you add/remove/edit a voucher (using the database option) on one server, you will need to do /vouchers reload to sync the current server with the database.

Also added an update checker.