Initial thoughts on a money composite

//ref #19 #20 

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Been looking at it.

```python
>>> tab.cash
USD 10

>>> tab.cash *= Decimal("2.1")
USD 21.0

>>> tab.cash.amount
Decimal('21.0')

>>> tab.cash.currency
'USD'
```

Currently it looks pretty bad from a usability standpoint but it "works".
This could probably be reduced to (with a helper method or something):

```
cash = money_composite(name_amount='_cash_amount', name_currency='_cash_currency')
```