New hook to determine if activity should be triggered

We should be able to tell activity not to create an activity message via a hook, like:

function mymodule_activity_comment($object)
{
      if ($object->cid < 100)
          return FALSE;

   return TRUE;
}

function mymodule_activity_nodeapi($object)
{
      if ($object->nid < 100)
          return FALSE;

   return TRUE;
}

function mymodule_activity_user($object)
{
      if ($object->uid < 100)
          return FALSE;

   return TRUE;
}