Group Combat improvements

* The first kill to join the group now spreads experience to the other
group members.

* A message will inform you when you get experience for a non-group kill
that still shares. For example, if someone killsteals you, or if you're
both fighting the same monster but not grouped.

* The message for group experience is only sent if you actually gain
experience. You won't see a message for mobs too weak to give you
experience.