List balances for the last 12 months

[sebinsua]

In order to support teller list balances business --interval=monthly --only-numbers we need to be able to:

1. Get the current balance.
2. Get the last 12 months of transactions.
3. Get the final balance of each month, including the current month. To do this we need to minus each of the transactions from the current balance cumulatively.
4. Create a Vec of month_final_balances.
5. Write this to stdout.

Note: if Teller adds from and to query strings this would be easier