field_currency_name fix.

Ok, guys :) I've got your idea. You freeze currency value for all monetized field values for ALL instances of the model. I needed to keep different currencies for values of the same column of different instances, so had to a lil bit modify.

Feel free to reject, but the way proposed things will get more fliexible to configure. It would be better to add another parameter with appropriate name to allow developers to specify instance-level columns' currencies by setting name of the column containing currency code.