Implement permissions

At this time the ad module has four global permissions: administer advertisements, create advertisements, edit own advertisements, and show advertisements. These are all assigned on a per-role basis. The ad module requires a finer-grained permissions system, allowing for example multiple people to own an advertisement, and of them some with complete control and others that can only view some statistics, some which receive notifications, etc.

It would be worth exploring existing permissions systems provided through Drupal contrib before creating a custom permissions system. Any suggestions are welcome.