Improve handling of single-letter currencies

[kmike]

Some of the single-letter currencies are not picked up:

In [7]: Price.fromstring('R273.00')
Out[7]: Price(amount=Decimal('273.00'), currency=None)

In [9]: Price.fromstring('61.858 L')  # Romanian New Leu
Out[9]: Price(amount=Decimal('61858'), currency=None)

We should support them. The main issue is that they're ambiguous; we should be careful not to pick e.g. L or R letter from the extra text appearing in price string passed.