Implement the correct logic for the balance view

The balance now has a correct logic due to the fact
that:

-the balance gets the expenses and accumulated money
from the date that the budget was set and not from the
beginning of the month
-the budget class now saves the date on which it is set
every time a new budget is entered
-if the new month starts - then the budget setting date
is reset to the first day of the month in order to get
the correct balance in the next month