Convert currency when posting back to AK. For now, we convert everything...

... to usd and specify currency in the action_charge_currency variable
Sorry about the trickiness. Will add comments. What this method does is returns a string representation of a charge in the format you'd expect it to be in. So yen would be just be a string form of that original amount, whereas dollars would be cents/100 and formatted to 2 decimal places.