Req3: Currency can add.  Added exception but assert_raise won't work.
Req 4: Currency can subtract
Req 5: Currency can multiply
Req 6/7: Hash of values added to currency_converter. Just USD and EUR right now.
Req 8: Converter returns the same currency when the same currency code is given
Req 9: Converter converts between USD/EUR correctly
Req 10/11: Converter holds more than two currencies, converts them all