Examples of usage:
>>> get_exchange_rates(current.USD, [current.EUR, current.RUB], datetime.date(2006, 12, 29))
{u'date': u'2006-12-29', u'base': u'USD', u'rates': {u'RUB': 26.333, u'EUR': 0.7593}}
>>>> get_one(current.USD, current.EUR, datetime.date(2000, 01, 03))
0.99108
>>> eur_usd(datetime.date(2000, 01, 03))
1.009