Retrieves the historic exchange rates from Open Exchange Rates site.

• date is a date returned by Date.new/3 or any struct with the elements :year, :month and :day.

• config is the retrieval configuration. When invoked from the exchange rates services this will be the config returned from Money.ExchangeRates.config/0

Returns:

• {:ok, rates} if the rates can be retrieved

• {:error, reason} if rates cannot be retrieved

Typically this function is called by the exchange rates retrieval service although it can be called outside that context as required.