Automatic update of the expense recipient balance

After an Expense has been created,  modified or deleted, the recipients
balance is updated automatically.

Same goes when an expense recipient is added or deleted
Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>
Expense.date defaults to today

Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>
typo

Signed-off-by: Balthazar Rouberol <brouberol@imap.cc>