Added expiration time for player's group
Add fallback group configuration
Optional expiration date for ranks

Usage: /setgroup <player> <group> [minutes]
Minutes specifies the amount of minutes the user will be allowed to keep their
rank. This is used to calculate an expiration date. It is not activity time.

Configuration option: groups.groupname.info.fallback
  Group to assign players to after the expiration of their rank.
  EX: group moderator has fallback set to builder. When players with a
  temporary moderator rank expire, they will be set to builder.