Refactor code of financial app operations

- Improve the way to create and edit account transactions.
    Now, register correctly the incoming and outgoing account transactions
    (Double-entry bookkeeping system - "Método das Partidas Dobradas" in
    portuguese). Once the outgoing values always will be referenced by
    source account.

- Rather than use positve/negative values, register in database only
absolute values and identifify transaction by the column operation_type(debit/credit).

Change-Id: Ic966cbbb7df787c2ce8512850af1329725f696ce