Implement new logic for counting budget length

Since the user will now be able to set their own budget start day - there
is a need to figure out the new length of the budget.

 -Implement SetDailyBudget method to shorten the methods in the Budget
 class.
 -Implement getBudgetLength method to shorten the methos in the Budget
 class.
Implement getExpensesBetweenDates method

- Add a method that will get expenses between two dates (dates between start
and end of the budget that is).