transaction comment is parsed as a posting

I assume this parser is quite young/relatively untested.. here's the first issue I've found. It fails to parse this entry from ledger/test/input/demo.ledger (because the comment is parsed as an amountless posting):
```
2011/01/14 Bank
  ; Regular monthly savings transfer
  Assets:Savings                     $ 300.00
  Assets:Checking
```