#3 Add support for ladders

[Alf Mikula]

Add support for ladders, with the following features:

1. Requirement that all ladder games should occur within a fixed radius of a particular location, eg., within 10 miles of a sponsoring Go club
2. Configurable handicapping rules (ie, correct handicap applies, with no more than 3 handicap stones in a game).
3. A ladder administrator is solely responsible for adding / removing players from the ladder.
4. Ladder challenges should be configurable, ie, you may challenge up to three rungs above you on the ladder.