add support for currency with 0 CurrencyExponent (JPY)

This is to remove the hard coded '2' and replace it with a look up.

http://download.chasepaymentech.com/docs/orbital/orbital_gateway_xml_interface_specification_5_8.pdf (p264-p265)

Currently when trying to charge naively in JPY we get this error:

    Error currency [392] with exponent [2] not supported by this merchant [NNNNNN].

By setting the currency exponent to 0, we get the desired effect.