WIP : CFD-144 log group activity

Modules to support logging activity in the message table for:
* create/edit/delete node content.
* create/edit/delete comments.
* activities about users and group membership.
This seems like it's an API function, so it shouldn't be prefixed with ``_``

also, if we really wanted to make it generic, I think it should be ``c4m_activity_node_get_group($entity_type, $entity)``