Submitting comments should also increase node radioactivity

Currently, node popularity metric (based on viewing nodes) can be implemented. The concept should be extended so that commenting a node would also increase its energy. Furthermore, the amount of energy increased by viewing or commenting a node should be configurable per decay profile. Then it would be possible to use the module to create metric for commenting activity. I.e. it would be possible to create a 'most commented nodes' view.