capital_gains_classifier fails when price is 0

[scanta2]

def reductions(entry):
        return [p for p in entry.postings if (p.cost and p.units.number and p.price)]

should be

def reductions(entry):
        return [p for p in entry.postings if (p.cost and p.units.number and p.price != None)]

or something like that (e.g. isinstance Amount). When price is 0, the if statement returns False.

[redstreet]

If you don't mind sending a PR with a unit test case, that'd make it a lot easier for me :)