Create dailyBudgetSum(), change initial check

Due to the fact that now the logic is based around budget starting and
ending dates, which can be basically random, the old formula for the
dailyBudgetSum doesn't work anymore.

Also the initial check should only check for the
DAY_OF_MONTH_BUDGET_STARTS key, because that's the only key that is needed
for the figureOutBudgetStartEnd method.

- Implement the getDailyBudgetSum() method.

- Implement a check with DAY_OF_MONTH_BUDGET_STARTS key and provide a
  default key for it, in case the user doesn't have it. This should
  suffice the figureOutBudgetStartEnd method.