Added ply.GetCreditCount() method. Returns number of credits.
Added ply.SetCreditCount(credits:Number) method. Sets the credits to a certain amount.
Added ply.ModifyCreditCount(amount:Number) method. Adds to credit count of a player, instead of setting the value directly. Accepts positive numbers (e.g. 6) and negative numbers (to subtract from credit count, e.g. -6)