Re: [webpayments] Expression of monetary amounts (#40)

I would suggest using ISO20022 instead of ISO8583 as 20022 is newer. Currencies should be defined using ISO4217.

See http://www.iso20022.org/standardsrepository/public/wqt/Description/mx/dico/rules/_YYB_8dp-Ed-ak6NoX_4Aeg_1337619430

ISO20022 differentiates between a fractional digit portion and further states that the separator is a dot (addressing concerns of non standard separators somewhat)

So in short, a string type, using a dot as the separator would be compliant and I would support this approach from both a standardisation and an ease of use perspective.

One complication is that is doesn't appear to allow micro-payments as ISO20022 states the "number of fractional digits must comply with ISO 4217". This implies to me that, for example in USD an amount of "100.001" would be non-compliant as the fractional part is smaller that the ISO4217 exponent for USD which is 2. 

Furthermore ISO20022 states the fractional part can only be 5 digits, so even if we relaxed the ISO4217 limitation, the smallest micro-payment that could be made would be "0.0001" or 1/100th of a cent if using USD as the currency.

I'm not expert on these standards, so would appreciate Vincent or Kris's view of my interpretation of the standards and how they might feel about some revisions to them to support micro-payments.

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