[currency] List parsing errors

[Stefano Rivera]

18:27 <tumbleweed> convert 2750 km^3 to kl
18:27 <Dazzle> 2750 km^3 = 2.75e+12 kl

2009-05-14 14:18:55,442 ERROR core.dispatcher: Exception occured in Currency processor of lookup plugin
Traceback (most recent call last):
  File "/home/dazzle/ibid/dazzle/ibid/core.py", line 23, in _process
    processor.process(event)
  File "/home/dazzle/ibid/dazzle/ibid/plugins/__init__.py", line 55, in process
    method(event, *match.groups())
  File "/home/dazzle/ibid/dazzle/ibid/plugins/lookup.py", line 339, in exchange
    canonical_frm = self._resolve_currency(frm)
  File "/home/dazzle/ibid/dazzle/ibid/plugins/lookup.py", line 314, in _resolve_currency
    name = self.strip_currency_re.match(name).group(1).lower()
AttributeError: 'NoneType' object has no attribute 'group'