Issue #31

I fix issue #31 (Zero items in unpaid bill list).  First, I use the
balance property to filter all paid invoices that have a balance of
zero.  The balance property uses rounding with the external number of
fraction digits.

Then, I implement a Spock specification for this case, and, at last, I
display the balance property in listUnpaidBills.gsp instead of
computing the balance for myself.