Significantly updated LadderPlayer, including adding biasing.

- Renamed ladderPlayerTest to LadderPlayerTest.
- Removed code already inherited from Lgrf2Player.
- Modified ladder finder to use getVacantNeighborCount() instead of
getLiberties() (which returns 0 when called with a vacant point).
- Made playOutLadders() work for ladders of both colors.
- Fixed a bug that caused some ladders to play out even after they were
over.
- Added biasing to the root node.