Refactor add_business_days and subtract_business_days

`add_business_days` and `subtract_business_days` are implemented by
repeatedly applying `next_business_day` and `previous_business_day`
respectively.

The code is identical. This refactor makes the intent  more explicit, improving readability.

lib/business/calendar.rb

@@ -129,10 +129,7 @@ def previous_business_day(date)
     def add_business_days(date, delta)
       date = roll_forward(date)
       delta.times do
-        loop do
-          date += day_interval_for(date)
-          break date if business_day?(date)
-        end
+        date = next_business_day(date)
       end
       date
     end
@@ -145,10 +142,7 @@ def add_business_days(date, delta)
     def subtract_business_days(date, delta)
       date = roll_backward(date)
       delta.times do
-        loop do
-          date -= day_interval_for(date)
-          break date if business_day?(date)
-        end
+        date = previous_business_day(date)
       end
       date
     end