Added ability to create an inline validation rule.

Not sure if there is anything like this but I didn't see anything. Sometimes there are really weird rules that need one off scenarios, instead of making an IFieldValidationRule, it would be nice to just inline it:


    Property(x => x.FirstDate)
                .Custom(x => 
                 {
                       var lastDate = x.ThirdDate.AddDays(1);
                       return x.FirstDate > x.SecondDate > x.ThirdDate;
                 }, StringToken.FromKeyString("First day must be greater then second and at"
                    + "least a day after the third."));