Add support for Mollom

Simplenews Mollom integration has been requested several times in the Mollom issue queue. Mollom provides an API to enable other nodes to be protectable via the Mollom configuration interface. http://cgit.drupalcode.org/mollom/tree/mollom.api.php

In our queue, there has been a patch submitted already that would address some of these needs. I've added it here as a starting off point for the work in this module.