Added SMSG_GROUP_SET_LEADER
Cleanup
Added group opcodes
Added SMSG_BATTLEGROUND_PLAYER_JOINED
Added SMSG_RECEIVED_MAIL
Added CMSG_SEND_MAIL
Added CMSG_MAIL_CREATE_TEXT_ITEM
Added CMSG_MAIL_MARK_AS_READ
Update CMSG_MAIL_RETURN_TO_SENDER
Added CMSG_MAIL_TAKE_ITEM
Update CMSG_MAIL_TAKE_MONEY