Add currency symbol handling to Mario's sandbox numberConverter

I will contribute a subclass of NumberConverter that converts numbers into BigDecimals and properly handles converting currency values (as this is the primary need for a BigDecimal-returning NumberConverter).

Joe Sam Shirah has agreed to grant ASF the license for the currency handling (the currency parser requires a currency symbol, but we don't want end users to have to input currency symbols unless they want to), and he'll attach this code to this issue.