Add finer grained permission check when adding or removing players from a group.

[grandwazir]

Now checks if a player has either player.addgroup.name or player.removegroup.name before making the change. This creates basic promote/demote functionality.

[leagris]

I vote up your patch

This is exactly what I am looking for.

Use case:

Have per city groups.
I want the city officials to manage adding/removing members of their own city group.