Almost finished criticals.

- Renamed ImmoblizeEffect to ConditionEffect and added Condition enum.
- Added Until.Unspecified to handle the case of prone and modified
  UntilHelper.ToString() and its callers appropriately.
- Changed AddPeriod() to AddTrailingPeriod().
- Added HealHitPointsEffect, FlyEffect and ShiftEffect effect components.
- Added Where argument to EffectExpression.Creature.
- Added ScoreTypeHelper.ToString().