Updated the sources to allow for custom curreny

This would solve issue https://github.com/cantino/reckon/issues/7 and seems to work fine.

If you have any feedback, I would love to hear it. Ruby is and have never been my primary language so maybe this could have been solved differently.

It still defaults to a prefixed $ for currency to avoid breaking backwards-compatibility, but then adds the ability to arbitrarily change the currency to allow for anything to be added. Including time, stocks, cows, you name it! :)