Converts foreign to base currency

Currency conversions will happen on observation and historical pull
of order and cart events, when the currency used in the event does
not match the base currency.
This requires the Magento base currency to be set to the same
currency as the Jirafe site currency.
Refactored for clarity.
Update Data.php

Returns currency code instead of currency.
Merge pull request #43 from jirafe/currency-conversion

Converts foreign to base currency