[DONE] Groups and permissions refactor

[xoft]

I've just found out quite a huge problem with the ranks handling - there is a possibility for a mismatch for player UUIDs when the server is offline - some methods use the player's UUID, which is the offline one, others don't have access to the UUID (because they don't have the cPlayer object available) so they use the cMojangAPI class to look up the UUID, and they work with the online UUIDs. I'll need to fix this somehow, probably mostly in the Core.