Handle UUID player files semi-nicely in POIgen (14w10a+, 1.7.6+)

This pull request is a clone of #1079 opened against the snapshot branch instead of the master branch.

Currently, POIgen crashes trying to handle player POIs on 14w10a worlds, since the `world/players` folder has been moved to `world/playerdata`. This commit patches POIgen to check both folders. Unfortunately, there is currently no way to get the player name from the UUID and it doesn't appear to be stored anywhere, so we're stuck displaying the UUID until Mojang implements a UUID -> name API. This is definitely not ideal, but it's better than a crash.

This patch is fully backwards-compatible; that is, it has no effect on behaviour when handling a 14w08a-/1.7.5- world.