Added the option to configure the resource owner.

Since Doorkeeper allows the configuration of the resource owner of an access token, WineBouncer should also make his resource owner configurable. Since the entity that belongs to the access token is not always a `User`. With this pull request you can now specify the entity the access token belongs to. For example if the entity is an `Admin`, you can now define the resource owner as `Admin` with the configuration block `Admin.find(doorkeeper_access_token.resource_owner_id) if doorkeeper_access_token`